You missed the pop-up. Sign out of the App Store, restart, sign back in. Search GarageBand. When the pop-up appears, click "Download" not "Cancel." If it doesn't appear, Apple may have deprecated the CDN for 10.15.7. In that case, you must have a friend with an older Apple ID share it via Family Sharing.
"Download an older version of GarageBand? The current version requires macOS 11.0 or later, but you can download the last compatible version." Stack Exchange What if GarageBand isn't in your Purchase History? garageband catalina 10.15 7 download
“I can’t update,” Leo said, staring at his M-Audio keyboard like it was a dying relative on life support. You missed the pop-up