Note on word choices here: Is it problematic to use the informal "나" with the formal "ㅣㅂ니다"
Is it wrong to use 저는 in this case?
No that is actually correct. The creator of this exercise is very wrong by mixing honorifics.
I don't think you need the "나는" as it is implied when you don't have a subject.
@duolingo: you are teaching too many mistakes!
This sentence makes no sense in english...
I used 저는 and it corrected it to 나는??