"My friends are at the train station."
Translation:Мої друзі на вокзалі.
It's correct, but unfortunately we do not have enough manpower to add "знаходиться" to every sentence that has "is" in it. And it can't be done automatically because we don't have any sort of smart algorithm that can find "is", decide that it can be translated as "знаходиться" (because not every "is" can), find the subject and conjugate "знаходиться" accordingly...
So, good for you that you know this expression :) But here it's not accepted, sorry. Simply write your sentence using "to be", I hope it's not too hard to remember.