1. Go to settings/music on your device and make sure you are signed out of 'iCloud Music Library’ and 'Apple Music'
2. Go to settings/iTunes & App Store and make sure you are signed out of your appleID.
3. Go to the music app on your device and open your profile (icon in top left hand corner of screen) and make sure you are signed out there too.
4. next go to your PC and open up iTunes and go onto the general tab in preferences and make sure the check bix to use iCloud is ticked ... You may want to untick it and apply the settings and tick it again and reapply the setting, if it was set already.
next go back to your device...
5. Go to settings/iTunes & App Store and sign into your appleID.
6. Go to settings/music on your device and check you have both the 'iCloud Music Library’ and 'Apple Music' switched on ... If not turn them both on.
7. Now go back to settings/iTunes & App Store (this is important) and make sure automatic downloads are turned on for music/apps/books etc. also turn on the 'use mobile data' setting too, if you want to stream/download music over your cellular network.
Now open up your music app on your device and make sure you are definitely signed into your profile and then see if the app now works okay.