"I do not eat chicken because I want fish."
Translation:Non mangio pollo perché voglio pesce.
Well, no, not for DuoLingo because they haven't taught us the conditional at this point and technically it would translate as "I would like" instead of the "I want" that they asked for.
But in actual speech, "Vorrei" is more commonly used when asking for something because it sounds more polite. As Pink CathodeCat said, "voglio" is more demanding.
"I want all your money." "Voglio tutti i tuoi soldi.".
"Vorrei" is softer and kinder. "Vorrei un menu, per favore." "I would like a menu, please." But again, that is NOT the tense asked for in this question.
It is wrong because, in Italian, the word "non" goes IN FRONT of the verb. "I do not eat" = "Io non mangio". "Non capisco" = " I do not understand", etc.
You don't have to include the word "io" because with "mangio" and "voglio", it is understood (but Duolingo would not count that against you. It is technically correct but the pronoun is often only included for emphasis).
You need an accent on "perchè" (also a minor error so not a failure).
I also think that you don't have to include the "il" with "pollo" and "pesce" but I am not sure if they would count that as an error. The English sentence didn't use "the chicken" or "the fish" but sometimes Italian uses an article when English doesn't. It doesn't look like you need it here. Best wishes.