Ok
1
try restarting your device by pressing the power button then slide to power off wait 30 seconds then power on then try again
2 sign out of your Apple ID by going into settings then tap the name at the top of the screen, then try to sign in and add songs
3 check if your subscription has expired by opening settings then press subscriptions if it has expired it will be down the bottom with the expired subscriptions also expired subscriptions could expire cause of invalid payment info so update it if that’s the case
4 if none if the above options work please contact apple support
i Wish you the best of luck
Dan