I can't open your assembly because you have a newer version than I do. However, Over defining means that you already have mates that constrain some object and adding that mate would "over define" it.
Also, planes do not always align by definition such as top to top or front to front. It depends on which plane you created the object. The "right" plane of your table may actually be the "front" depending on how the geometry was created. Just try mating it to one of the other planes and see if it aligns correctly.