"Corinna is not a girl, but a woman."
Translation:Corinna non est puella, sed femina est.
See the course introduction: https://www.duolingo.com/skill/la/Introduction/tips-and-notes
There's nothing wrong with your rendition. Sometime not all possible word orders are accounted for on Duolingo. Do note, though, that in copulative clauses ("X is Y") est tends to come in the middle. When a verb is repeated in a subsequent clause, it is regularly omitted:
Corinna nōn est puella, sed femina.