Verification Failed: An unknown error occurred

I upgraded to macOS Catalina, Version 10.15.4, on my Mac mini (Late 2012), and now I am unable to connect to the to the App store. I get the error message, "An unknown error occurred." I had no problems with updating third party software to work with the new OS, but I cannot update any Apple apps. I'm sure the problem is in my computer, but where?