Hey ginza63,
We see that you're receiving an error 'GarageBand was not able to complete the download', and we'd like to help!
Have you tried force quitting the GarageBand app, and re-opening it? How to force an app to quit on your Mac
We also recommend that you try restarting your Mac to see if that helps.
If you continue to experience the same issue, try deleting the GarageBand app by following the steps from here: How to delete apps on your Mac
Deleting the app won't delete the files from GarageBand. The saved files are saved under another folder, and you're only deleting the app itself.
You can reinstall the GarageBand app through the App Store: Download apps and games from the App Store
We hope this helps!
Best regards.