I think no dictionaries list "wishlist" as a single word. It is an error, but I don't doubt that it is becoming more common. Still I think no dictionaries have recognized it as an English word yet, so neither do we. The exercises for this sentence in Duolingo do not show "wishlist" as an accepted answer. It's possible that Duolingo accepts the answer with the missing space, but it should certainly never show it to you without the space. However, since this course is teaching Klingon and not English, I will enter it as an accepted variation.
It did show it to me without the space, though. In the exercise where you choose between the three words with the pictures. I checked the lesson review at the end, and it definitely said "a wishlist". (This was the last lesson of "Home Life".) It was the inconsistency that bothered me, more than any particularly strong feeling about whether it should be accepted in principle.