Clueless? No.

Even herbivores are deadly when threatened. The law of nature is a dog-eat-dog world, without that kind of pressure, evolution is impossible. At best, a species may be able to get away with hiding, but once a species develop intelligence, they start requiring things which can't be hidden, like smoke from fire. Pacifism is a luxury of those with weapons or defenses.

However, it's very possible that they'd be able to form a society without the level of fighting that humans have done if they're psychology is distributed more towards a defensive culture and a strong mental block against fighting. The things about combat is that a small group willing to fight can cause everyone else to respond in kind, or be conquered, so they would have to make defenses - but offensive strategy may be mostly unknown to them.

That being the case, they'd have no experience with offensive combat - compared to humans, which their observations would reveal that large swaths of the population are constantly engaged in mock wargames with each other. And they've even programmed futuristic military simulations, the primary one called 'Starcraft II'. Humans sound like a great choice.

Sure. It's even possible to be very familiar with violence and not be good at warfare; think about the difference between a warrior and a soldier (current US military propaganda notwithstanding): a warrior is an individual. A soldier, by definition, is one element of a fighting group. Being good at one doesn't automatically mean being any good at the other. You could imagine a species where singular dominance battles are common yet the idea of ganging up in an organized fashion beyond temporary and immediate groupings of convenience never took hold.

And it starts at the basics: modern military training and indoctrination is the result of decades and even centuries of hard-earned experience in what works and what doesn't, and it's always evolving and being refined to take into account cultural, societal, and technological changes. It's not something that can be avoided.

A good example is the American Revolution. Historical myths to the contrary, it wasn't a bunch of freedom-loving non-military farmers who took up arms and defeated the British Forces out of pure patriotism and the power of liberty; just about all the British military defeats relied on the colonists eventually fielding trained forces. Even Lexington and Concorde. Historical research has shown that a great many of the colonists were in fact veterans of the French and Indian (or Seven Years) War, and thus trained troops; if anything it was the British at that fight who were inexperienced, many not having previously been in a conflict. But on a larger scale, the Americans spent much of the early part of the war in constant retreat until they'd accumulated a cadre of battle-experienced troops and imposed traditional European-style discipline and training so they could have a chance in open battle.

Which Washington, incidentally, was well aware of; an experienced soldier himself, he had a low opinion of amateur militias based on his own experience. The army that took to the field at Yorktown was a professional, conventional one. And the same lesson had to be learned in 1812. And 1861. At least by 1917 they figured out you couldn't throw untrained troops into the meatgrinder of modern warfare and expect them to survive with only the love of liberty protecting them. Turns out that isn't very bulletproof.

Not that this is a uniquely American thing; the Russian communists had to figure out the same lesson. It took Trotsky reorganizing (well, creating, essentially) the Red Army, employing experienced former Czarist officers and non-coms to train the troops, before it became really effective on the battlefield, having realized revolutionary zeal didn't overcome aimed firepower and professional military tactics.

You'll note in those two example technology wasn't the important factor. In both cases the two sides had at least equivalent weapons. Both sides had literate leadership capable of reading and knowing the theory of successful warfare based on history and the experience of others, but it took until they had troops drilling on the parade ground with sergeants bellowing at them, mattresses being tossed in the barracks for not having the corners done correctly, and people doing pushups until they puked until they got it into their heads why you only pointed the weapons downrange that they could get on the battlefield and be expected to win and not have to rely on luck or the enemy screwing up.

Relevant to your question, in both cases these are human groups, in real life who had to learn that lesson, and you still see it happening today. People who think that because they've got a military weapon they can take on a military force. Give me a bunch of yahoo "militiamen" and a company of Marines (US, Royal doesn't matter) or from just about any other professional military with the same weapons but half the size, and they'll cut through the "militia" like a hot knife through butter the vast majority of the time except in very specific circumstances. And again, these are people who have access to lots of history and documentation demonstrating over and over why some things work and some don't, and yet that information simply isn't incorporated, despite example after example why it has to be.

So that being the case with humans in reality, it's hard to argue that aliens in fiction couldn't have that same sort of problem. The difference being, in the case proposed, the aliens are smart enough to realize they have the problem.

Think about experience.

Imagine a society with generations of pacifism. They have no generals. They have no weapons or battleships. They might understand the concepts of waging war, but it will still take many many years for them to train generals, or build the infrastructure necessary to create weapons of war.

How long do you think it takes to train a general?

How long do you think it takes to create the infrastructure necessary to construct a battleship?

Your non-human society is going to be looking for those with experience. It's unlikely that they've never seen violence (think about the conflicts in the natural world), or they haven't read stories and tales about battle (think fiction), but they need the expertise of humans who have lived through these conflicts before.

They evolved with "Us against the Environment" (which requires cooperation and technology), not "Us against Them" (which requires warfare)

Evolution of intelligence (and hence technological advancement)

Many theories of the evolution of intelligence (in humans and animals) rely on a machiavellian drive. For example if a species has complex social interactions (e.g. chimpanzees) they need to learn who's who, who's dominant, changes in dominance hierarchy, how to deceive others to obtain food, getting and maintaining allies, forming "war" parties to fight other rivel groups etc. But there are other ways we can see intelligence developing - like extractive foraging (you need tools and techniques to get food - see New Caledonian Crows) or omnivory (learning what is safe to eat and where to find it - see rats) or pair bonding/social group dynamics (and the complex social interactions that result from that - see parrots and elephants). In fact when we think of animals that most scientists consider intelligent many are relatively peaceful and pro-social, like parrots, elephants, whales, rats, octopuses. Even among great apes, orangutans, gorrilas, and bonobos are relatively conflict free (besides fighting over mates/territory). So we can evolve intelligence without the need for self-serving conflict but is conflict the inevitable result of society?

Society without war?

Most conflict is about resources (food, shelter, mates etc). One could argue that conflict can also be about ideals/culture (holy wars, genocide etc) but even those conflicts are probably underpinned by the perceived threat of resource shortage. If there are enough resources its just not worth your time, effort, or risk of injury to fight. So is it possible to have a society without inevitable resource shortage? What about an environment that is so risky, populations never get big enough to have resource shortage and conflict - e.g. poison gasses frequently spill from underground in unpredictable places killing large parts of the population off; some type of radiation or something from space; a rapidly evolving pathogen that the society (despite their technology) has not been able to purge but keeps baseline population low. These challenges you cant fight with war but with technology and cooperation. Once the species becomes space-faring they may no longer have to deal with the environmental threat but at this point they are having fewer offspring (like many nations today) and have more resources from other planets. Or perhaps that pathogen has followed them.

The aliens they are fighting are more similar to humans (and use human style tactics, warfare, and psychology), than who they usually fight against

Perhaps this alien species is not peaceful per se (they regularly fight within their world or with other alien species) BUT they have never encountered an enemy like this. Warfare changes a lot depending of the technology available, the terrain/environment, mindset/psychology of the people, motivations for the war etc. Think how damaging guerrilla warfare was when previous battles had been two large groups fighting head on. It would be good to learn about defending agains guerrilla warfare from people who are experienced with it rather than learn about it through trial and error. And what about the psychology of other aliens - I'd imagine the concept a kamikaze was not something that the USA predicted. Perhaps the aliens they are fighting are more like humans and they want to know what they are capable of, or where their weaknesses are (like demoralization or propaganda which may have no effect on their population)

Gene editing, MAD, and socialization

If they have advanced spacefaring technology long before they encountered another intelligent species, they may have almost completely conditioned themselves against war and war like thinking. To make it more believable perhaps they have engaged in large scale gene editing to make violence very difficult. possibly something like super-empathy. Above a certain level of technology going to war with your own species basically means the extinction of your species. Works even better if there is more than one habitable planet in their system so their ancestors had to make a choice about altering their behavior or going extinct through interplanetary conflict. It helps if they had a close call in their history, a terrorist cuban missile crisis with FTL technology (to stretch a analogy), something that made the realize one lunatic could wipe out their entire species.

They can reinsert the genes but they have lost all knowledge in how to socialize these violent children, so they keep ending up with hyper-aggressive lunatics that are far more dangerous than the enemy. So they need a species that is still knows how to socialize potentially violent people.

The supercolony.

Consider the Argentine ant. In its native lands, different ant colonies fight each other. But in the course of invading new lands, this ant has formed a supercolony. It is something different.

https://en.wikipedia.org/wiki/Argentine_ant

According to research published in Insectes Sociaux in 2009, it was
discovered that ants from three Argentine ant supercolonies in
America, Europe, and Japan, that were previously thought to be
separate, were in fact most likely to be genetically related. The
three colonies in question were one in Europe, stretching 6,000 km
(3,700 mi) along the Mediterranean coast, the "Californian large"
colony, stretching 900 km (560 mi) along the coast of California, and
a third on the west coast of Japan.

Based on a similarity in the chemical profile of hydrocarbons on the
cuticles of the ants from each colony, and on the ants' non-aggressive
and grooming behaviour when interacting, compared to their behaviour
when mixing with ants from other super-colonies from the coast of
Catalonia in Spain and from Kobe in Japan, researchers concluded that
the three colonies studied actually represented a single global
super-colony.

The researchers stated that "enormous extent of this population is
paralleled only by human society", and had probably been spread and
maintained by human travel.

Ants from the supercolony will not fight each other. Any part of the supercolony is as good as any other part. The ants can be aggressive against food sources and defending the nest against predators. But "war" means organized aggression against conspecifics and these ants have dispensed with war. In doing so they are on their way to conquering the ant world. In Southern California, it is hard to find any other ants but these.

So too your aliens. Through cooperation they have conquered their world. They have no experience making war on an intelligent adversary.

Humans: be very careful what you teach these creatures.

You don't neccessarily need them to be completly void of war in their history for them to be really bad at modern war.

They may have fought wars in their past, but that was 4000 years ago when they were in the technological equivalent of roman or maybe renaissance technology. And due to (poltical development/religion/different evolutionary psychology, etc.) they managed to not have wars since.

So now that war has come to them they are kind of helpless and don't know how to deal with the situation. They managed to design some pretty good plasma rifles and they got a million of them rolling of the assembly lines every day - after all that is just a bit of high-energy physics, metallurgy, mechanical engineering and mass production - things they are doing everyday anyways, but military theory takes time and experimantation and iteration and that's something they don't have. So they could try to figure things out on their own and suffer horrible losses in the meantime, or they could just ask the humans and we give them general staffs, military ranks, combined arms, medical and engineering corps, division, and all the other nice stuff it took decades or centuries to figure out immediately.