Wrong feedback on practice
Perhaps it meant to say that it should have been 동불은 instead of 동불이 (which should be accepted as well). I (obviously) can't tell if that is an accepted sentence in this case. From what I can tell, that is the only difference between the two answers.
Firefox 59.0.1, Windows 10, Time Practice.
Edit: Yes, 동불은 is accepted.
It's saying your answer, 동불이, is not accepted. If you are sure it's correct then use the "Report" button to report the error so that the course contributors will see it. They don't see all the posts and comments in the sentence discussions or discussion forums.
If you're not certain that it's correct, then use the "Discuss" button, see if this has been discussed yet, and create a new comment or post if necessary.