Before downloading, ensure you have enough storage space. GarageBand is a large application; the initial download is usually over 1GB, and the "Sound Library" (instruments and loops) can take up anywhere from 10GB to 30GB depending on how much content you choose to install.
Follow the steps in Method 1 to download the "last compatible version." Method 3: Direct Downloads (The Last Resort) garageband catalina 10.15 7 download
"The current version requires macOS 11.0 or later, but you can download the last compatible version." to install version 10.3.5. Stack Exchange Workaround: If GarageBand is NOT in your Purchase History Before downloading, ensure you have enough storage space
Once you have the older version installed, it will work smoothly on 10.15.7. it will work smoothly on 10.15.7.