How can I download an older version of Garageband since my Mac is not compatible with Mac OS Big Sur ?

Since Apple introduced the Mac AppStore we have to grab any GarageBand version while it is available for our system version. There has always only the most recent release been available. Usually the current version is compatible with last years version of macOS as well, but the current version is special, because it has been released as a dedicated unreal binary for Big Sur, optimised for Apple Silicon M1 and Intel Macs with Big Sur. It cannot even run on macOS 10.15 Catalina. You may want to contact Apple Support and ask for help to get the previous version.

There should have come a compatible version of Garageband with your Mac. Perhaps you have still a copy in an older backup? GarageBand does not need an installer. You can simply copy the application back and then try to download the sound library again.