Had the same problem. In the bottom toolbar, click the icon just to the right of "My Music" (unfortunately, I can't remember what it said but I think it was a heart icon). On this screen, it will ask you if you want experts at Apple to pick music just for you. Select Yes or No. If you select No, the heart icon goes away and your music now appears on "My Music" list. This is probably a bug in the Music app.