How to fix the App Store after upgrading to macOS Catalina?
Solution 1:
What I have done is delete the App Store key chain entry in Finder, Application, Utilities, Keychain Access and the search for appstore. Delete just that one. (MacOS Catalina)
Solution 2:
After trying every other step, including reinstalling the catalina os from recovery mode
Only this worked for me
keychainaccess -> preferences-> clicking on "reset my Default key chain" -> reboot machine worked as stated here
Note: you will loose all the saved passwords