how can i upgrade GarageBand for catalina

You can’t. GarageBand is now apparently Big Sur only. If you have GarageBand installed you can keep using it of course. It’s not going to just stop working you know. But as for further upgrades it appears going forward that GarageBand upgrades will be for Big Sur and later only.

Unless, that is, if this situation is actually a glitch in the App Store. But it’s been two weeks now and not a word from Apple about it or even an acknowledgement of the issue.

Which version of GarageBand do you currently have installed? If it worked on your previous system it will hopefully work on Catalina as well. The most recent version at the AppStore is in the new "universal" format for intel Macs and Apple Silicon Macs and requires Big Sur. The AppStore should not offer this upgrade for users with Catalina installed, since the only advantage is, that the version is making better use of Apple Silicon and is optimised for Big Sur. It has draw backs as well. The new GarageBand does no longer support the Media Browser and it is more work to import songs from the Music.app.