Translation:I got a good grade on the homework.
No-one would say 'gotten', surely. It's an Americanism. We should be encouraging learners to be accurate in both English and Welsh.
"Have gotten" is a perfectly fine way of saying "got", American English is just as correct as British English.
More correct in Duolingo's view, apparently, as the British English alternatives are assessed as 'wrong' and the sentence keeps recurring until the learner is forced to type 'gotten'.