Find a Mac running a newer OS (like Big Sur, Monterey, Ventura, or Sonoma).
If you cannot access a newer Mac and the App Store method is unavailable, online software preservation archives are an alternative. Websites like the Internet Archive track older, legitimate versions of Apple software packages (.pkg files). Important Considerations: Garageband For Catalina 10.15.7 -FREE- Download
Go back to your Catalina Mac. GarageBand will now be in your "Purchased" list, and the trick above will work. Why go through the trouble? Find a Mac running a newer OS (like