"Apakah dia sudah menyelesaikan sekolahnya?"
Translation:Has she already finished her school?
Yes exactly. “Sekolahnya” in the Indonesian sentence refers to the process of her education, not the place/building in which she is educated.
Formal and most literal translation: “Has she already finished her schooling?”
Informal and most natural translation: “Has she already finished school?”
The translation given by Duolingo is incorrect because it uses “school” as a place/building in the English sentence and “sekolahnya” as the process of her being educated/schooled in the Indonesian sentence.