"a big garage"
Duo explains this in the Tips for this level. As I understood it, the absence of a definite article (the), implies the indefinite article ("a" or "an"). Without it, you'd be saying something like, "I have big garage" as opposed to "I have a big garage." You need to include "a" in the answer so Duo knows you're learning this. We haven't learned "the" yet, so if nothing else is there, you can (and need to) assume an indefinite article. Someone please correct me if I'm off track here.
When you get a short phrase wrong, you should be able to hear it again correctly. Getting a wrong answer and seeing the senetence in English and having just heard the last thing you entered (the wrong answer) is kind of confusing. I understand that the same question might be asked again in a different form right away- but it would help a lot to validate the correct way of saying a word after answering- especially when answering wrong.