[SOLVED] I have had the same problem (March 2021). I'm running Mac OS Catalina 10.15.7. Since my iMac can't be updated to Big Sur, I decided to try to delete Garageband from my iMac and reinstall it. But to reinstall you have to go on the App Store in your computer and then click under your user name and then you'll see all the apps you have previously downloaded or bought. Click to download the last version of Garaband you had (which is the one that will be available to you. Don't try to download garage band searching the store because the version that will be there will be the one made only for mac OS 11. That worked for me.