MacOS – How to fix the App Store after upgrading to macOS Catalina

Got this sorted out.

1. Quit App Store.

2. Use Terminal or Activity Monitor to quit/ force quit the following processes and to delete the following folders:

    killall storeaccountd
    killall storeassetd
    killall storedownloadd
    killall storeinappd
    killall appstoreagent
    rm -rf ~/Library/Caches/storeaccountd
    rm -rf ~/Library/Caches/storeassetd
    rm -rf ~/Library/Caches/storedownloadd
    rm -rf ~/Library/Caches/storeinappd
   

3. Restart App Store and hold the option key down to cancel any stuck downloads.

4. Then try a download again

Ultimately, I had to delete the Airmail app and reinstall it entirely. It's now able to update.

The problem appears to be due to the fact that when Airmail 2 was upgraded to Airmail 3, the internal app version was 3 while the version shown in the Applications folder was v2. I think this confused App Store.