it happened to me as well because of iOS 8.4 update. as you noticed, after this update, you will see new-looked Music icon. and this Music app now works like Appstore / iTunes / iCloud. which means the Music app will use your appleID information. If you are using more than 2 apple ids (in my case, i had several appleid of other country), you may see that message if your apple id logged in Appstore is NOT same as Music app. So, you need to log out iTunes or Appstore AND Music app as well and run Music app (if Music app was running background, force close first) and sign in with the SAME appleID you want to use on Appstore / Itunes. Once you log in with appleID you want to use on Appstore, your appstore will be changed automatically with appleID. Hope your issue can be solved with my tip.😐 😐