This must be idiomatic usage because Benutzer is given in two dictionaries as the translation of 'user', also 'Benutzer is feminine, whilst on Duolingo 'Nutzer' is Masculine. Both dictionaries are 2012, one 'Collins' the other 'Oxford', so I think an explanation from Duolingo would be useful.
Both Nutzer and Benutzer mean user and the native speakers I know have struggled to come up with examples where one could be used but not the other.
You can confirm this be checking the translation of Nutzer in your preferred dictionaries.
I should also point out that both Nutzer and Benutzer are masculine nouns.