This post is designed to be helpful, safe (avoiding piracy), and solution-oriented for users stuck on older macOS versions.
Go back to your Catalina Mac. GarageBand will now appear in your Purchased history. garageband catalina 10.15 7 download
Thankfully, Apple allows users to download the last compatible version of software. This means that if you have an Apple ID with a purchase history (even free "purchases") tied to GarageBand, the App Store will offer you version 10.3.2—the final build that supports Catalina 10.15.7. This post is designed to be helpful, safe
If you have never "bought" GarageBand with your Apple ID before, it won't appear in your purchase list. You can solve this by: Using a Newer Mac safe (avoiding piracy)