"No one wants to be pessimistic."
Translation:אף אחד לא רוצה להיות פסימי.
אף אחד אינו רוצה להיות פסימי
You're never supposed to use לא for the present tense
It may sound formal, but it is the more correct version and is common in regular speech, even songs.
I guess it's OK for Duolingo to allow the colloquial version but the correct one should definitely be accepted. This is no אלונטית or שלחופה.
I agree the course should allow both, but I think it's important to know that if you use אינו when talking to people you will not sound natural. In song lyrics and written language, yes, I agree, but not in spoken language.