I really wish exercises like this were structured a bit better somehow, or had better hints, or something. For this one, it's pretty obvious what the statement means, but I'm not sure how you're supposed to know to type "I am fine." for "Met mij gaat het goed.". It just feels like you get these ones wrong even though you know what they mean because you don't know what phrase Duo is fishing for. I'd rather have a multiple choice question where this is one of three responses to a question or something like that.
Just my two cents.
They do. A lot of alternatives are accepted, but the course is in beta, so it's possible that some alternative translations aren't accepted yet. And just by looking at this discussion, you can see how many alternative ways there actually are to say I'm fine, it's easy for the course builders to miss a few. Luckily we can help them out by reporting these correct translations by using the report button right after entering an answer in an exercise.