"The girl does not eat this onion."
Translation:Το κορίτσι δεν τρώει αυτό το κρεμμύδι.
That's because you used the wrong article :P
Keep in mind that κορίτσι as a noun is neuter in gender, so το would be the correct article, and not η. If you use the wrong article, Duolingo marks your noun as incorrect and replaces it with an alternative that matches the article you used, if there is one (in this case, η κοπέλα.) ^.^