"The buses are not in the front, but in the back."
Translation:A buszok nem elöl vannak, hanem hátul.
elótt is a postposition that comes after a noun: X előtt is "in front of X".
elöl, on the other hand, is an adverb that stands alone.
Here, it is not specified what the busses are in front of, just that they are generally in front.
So just as you couldn't say "The busses are in front of" in English (it would prompt the question: in front of what? there's something missing), you can't use előtt by itself without a noun in Hungarian.