: Sign in to your Apple ID on a Mac running macOS 11 or later and "get" GarageBand. Once it is linked to your account, you can return to your Catalina machine and follow the "Purchased" steps above.
Before downloading GarageBand, ensure your Mac meets the system requirements:
If you previously downloaded GarageBand 10 on any Mac using your current Apple ID, you can easily reinstall the compatible version.
: Sign into your Apple ID on a different Mac that is running a newer OS (like Big Sur, Monterey, or Ventura). "Purchase" (it's free) GarageBand there to link it to your account. You don't even need to finish the download. Once it's in your history, return to your Catalina Mac and follow the "Purchase History" steps above.