Translation:Saturdays I play the violin at the restaurant.
Because prepositions are very arbitrary. The meanings of things like in, on, at, to, etc are very fuzzy, and you could easily argue for the use of several of them for any given application. But in a specific language, only one may be acceptable. So while in one language in the beach, at the beach, and on the beach are all fine, and near the beach would be different, in some other language you may be forced to only say one of them, or other things might work too.
I agree. Duo is inconsistent at times. I can see where it does not want us to say "every" if there is no Spanish word for "every," and I can see how it can't have possible "correct" answer in the database, but there are sooooooo many times it seems to reject the obvious and/or accept the odd, IMO.