A prompt will appear: "The current version requires macOS 11.0 or later, but you can download the last compatible version." . Click to install version 10.3.5. Method 2: If You Have NEVER Downloaded It
The only legitimate way to download a free, compatible version is through your existing Apple ID purchase history. The standard App Store "Get" button will often block you with an "Incompatible" error. Garageband For Catalina 10.15.7 -FREE- Download