"Tú vuelves a esta tienda a menudo."
Translation:You come back to this store often.
"You come to this store often" should be accepted. To come back is essentially redundant. By definition if you come to the store often, you are also coming back. English is a lot more vague in its meanings.
go back was used previously in this lesson and should be accepted as well as come back.
"a" means "to". And I think the concept of coming back and going back depends on the sentence. As this one says "this store" it makes more sense to say "come back" instead of "going back". If it said "that" store, then we should use go back.