"You want to win?" is not a complete sentence. It needs to the "do" to be grammatically correct in English.
I think it's fine in casual English to leave out the "do", especially when speaking to friends or informally, because when you speak you can hear the intonation indicating that it's a question. But Duolingo, as we all know, is particular about some answers... and the formal, correct way to phrase this question is with the auxiliary verb "do." It allows us to easily recognize that the sentence is a question, maybe similar to using "không" in Vietnamese, which we don't translate, but is necessary to clarify things.