"There is no butter."
«Нема» and «немає» mean the same thing. Both are grammatically correct, and both should be accepted.
Actually, English 'there' often acts as a dummy pronoun, not referring to any place in particular. You can't say "No butter is" or "Butter is no", so you need to re-arrange the sentence and include "there": "There is no butter". So, "there" can be a 'dummy' word to make the sentence grammatical, and therefore it's not translated.
Of course, without the context, we don't know if 'there' is a dummy word, or if it refers to some place. So, both «Там нема(є) масла» and «Нема(є) масла» should be accepted.