Csn you say Nie potrzybuję innego dieta?
Nie potrzebuję innej diety. But that would mean "I don't need another/a different diet."
Edit: I see now that 'innej diety' is also an accepted answer.
It should be: i do not want another diet
And it is indeed the starred answer.
Why "next" is bad. Next = następny
Although "next" -> "następny", "następny" does not always translate well into English as "next".
Could you tell me what your exact answer was?