Have you GarageBand with your Apple ID?

If you have previously downloaded GarageBand with your Apple ID, you can force the App Store to provide the last compatible version: on your Mac. Click on your Name/Account icon in the bottom-left corner. Manually scroll through your list of Purchased apps GarageBand Download icon (the cloud with an arrow) next to it. A prompt will appear stating:

This feature is Apple's built-in solution for users on older hardware or those who prefer not to upgrade their OS. When you try to download an app that is no longer compatible with your current OS version, the App Store will often present a prompt asking if you'd like to download an older, compatible version instead. This is the golden ticket for installing GarageBand on Catalina 10.15.7. The last compatible version for macOS Catalina is GarageBand 10.3.5.

10.15.7 is the final update for Catalina. As the "end of life" version, it is the most stable. GarageBand rarely crashes on this build if the project isn't overloaded. It is a "set it and forget it" reliability level.

: Ensure you are signed in with the Apple ID you have used previously.

Launch the app store. At the bottom of the page click on your account. Find Garageband in the list. Click the cloud download icon. Apple Support Community I want to install Garageband 10.3.5 on Catalina!

However, you can still get a compatible version by following these methods:

Finding and downloading the correct version of GarageBand for macOS Catalina 10.15.7 can be challenging. Because Apple frequently updates its software, the Mac App Store usually prompts you to download the latest version of GarageBand, which requires macOS Big Sur, Monterey, or newer.