Are you free tonight?
So oddly, I answered “Abend” with “Nacht” which made sense to me as the translation is for “night”. But, it replaced my answer with “Abend” and then counted it as correct. Does it just consider them equivalent or is the correct expression “Abend” and not “Nacht”?
Literally, you are right.
However, the use of these expressions is different in the two languages.
“Are you free tonight?” in English would be okay for e.g. in invitation for dinner.
No one in Germany would ask “Haben Sie heute Nacht Zeit?” for dinner. That yould only be appropriate for e.g. a hunting trip until the early morning .
So, for the overwhelming majority of occasions the translation with “Abend” is correct.
I’d like to add what happened here, technically.
It seems that in the source from which Clozemaster gets its content (the database Tatoeba), the sentence was translated with “Abend” because, as @anon94972132 said, no German would use “Nacht” (except if you want to meet after midnight; even 10 p.m. would still count as “Abend” or “Spätabend”, meaning late evening).
Clozemaster then added “Nacht” as an alternative answer to the sentence because Clozemaster accepts both answers. (Possibly because too many users complained that their translation wasn’t accepted, but this I don’t know.) Technically, whenever you give an alternative answer rather than the original answer, you will see the originally intended answer after solving the sentence.