Ensure you’re running the latest app from the App Store/Play Store, and also that you have updated the firmware (Settings - General Settings - System Updates - Check for Updates). 

Failing that, force quit the app and re-open it. 

Failing that, try adding YouTube Music from another device. 

Failing that, reboot the router (off at the wall for five mins), and try again.

Thanks.  I tried on my computer, and it worked.