The singular word is natt.
en natt 'a night'
natten 'the night'
nätterna 'the nights'.
Some words have this vowel change in the plural, it's what we call an umlaut in Swedish (we don't refer to the letters åäö with this term).
Here's a link to a discussion with a list of words like that.
I'm guessing it's a language thing. I know that in French to say "at night" you'd actually have to say "the night". So you're not technically wrong if you translate literally, but it makes more sense to give the more 'natural' translation, hence the correctness of both answers.