The App Store won't let you download it for the first time on Catalina. You’ll need a "workaround": Garageband & Catalina 10.15 - Apple Support Community
If you already purchased or previously downloaded GarageBand with the same Apple ID: Garageband For Catalina 10.15.7 -FREE- Download
Before downloading GarageBand, make sure your Mac meets the system requirements: The App Store won't let you download it
A pop-up will appear saying: "The current version requires macOS 11.0 or later, but you can download the last compatible version." Click . Garageband For Catalina 10.15.7 -FREE- Download