Clock alarm - internal error - SOLVED

If you click on "Pick a song" in the alarm clock settings, there comes up the message "internal error - the requested app extension could not be found".

A lot of people had the exactly same problem as me and they weren't solved:

Clock Alarm- Picking my own song - Apple Community

Internal error on alarm clock - Apple Community

iPhone alarm - Pick a song - Internal err… - Apple Community

pick a song for alarms gives me "internal… - Apple Community

I found the solution to this problem:

You have to (re-)install the app "Apple Music" for this to work.

I hope everyone can solve the problem this way.