Before downloading GarageBand, ensure your Mac meets the system requirements:
If GarageBand is not in your purchase history, you cannot download an older version directly on Catalina. You must first "own" the app on a newer system: Apple Support Community Garageband for Catalina 10.15.7 - Apple Communities garageband catalina 10.15 7 download
If you're having trouble downloading GarageBand from the Mac App Store, you can try the following: Before downloading GarageBand, ensure your Mac meets the