How to let my wife share my Music account?

It should all be very simple but it is not.
I have upgraded the music subscription to family so that my wife can distract herself during her dentist appointment. She hasn’t used Apple Music before and I thought she would be able to use it through the family activation account.
When starting the music app, the message “An error has occurred, try again” appears. “Radio” works.
Where is the easy way?

You need to setup Family Sharing.

Sharing is active, she uses my iCloud space to back up her iPad and iPhone.

If a simple restart of the phone doesn’t help, call Apple tech support and see if they can work out what’s wrong.