The Nintendo Account you used to log into Nintendo Music may not have an active Nintendo Switch Online membership associated with it.
Try logging out from the app and logging in with a different Nintendo Account that has an active Nintendo Switch Online membership.
To view your Nintendo Account's Nintendo Switch Online status, follow these steps.
1. Visit https://accounts.nintendo.com.
2. Select “Online Services".
3. Check "Nintendo Switch Online Membership Status".
If it states you do not have an active Nintendo Switch Online membership, please sign in with a different Nintendo Account and check the membership status of that account.
*The Nintendo Account you are logged into on your browser may be different from the one you have used for Nintendo Music.