Windows 10 is trying to update to 1903. During update I receive an Error telling me to Manually Uninstall Avast, winch is currently not installed and does not show up in my Apps & Features (or other utilities I use to uninstall programs/apps).
If I try to Install Avast I receive an AvastUI.exe Error: Code execution cannot proceed because JsonRpcServer.dll not found…
Hi
If I understand your post correctly, there may be Remnants from previous Avast installations
There have been other users with Windows Update issues/Errors
Read the Topic here (specifically, post #5 by BWhit)
Maybe the info will help 
https://forum.avast.com/index.php?topic=228979.msg1516600#msg1516600[/url]
Maybe you have residuals from a previous install of Avast? This should help,
Uninstall Avast
Download the Avast Uninstall Utility to your Deskto http://www.avast.com/en-gb/uninstall-utility
For Windows XP and Vista get the Avast Removal tool from here https://install.avcdn.avast.com/iavs9x-xp/avastclear.exe
Uninstall Avast via control panel and follow the directions.
Run the Uninstall Utility and accept the reboot to safe mode
Once complete reboot your system.
A video to help you use the Avast Uninstall Utility is available at: https://youtu.be/KD8LO8OxmvM
Just to point out, OP said Avast wasn’t even installed Bob.
I’m sure you read BWhits comments
The comments I think would suggest that if older version UI files were in different folder (for what ever reason) a Clean Install would not solve the issue BWhit has explained.
I’m not suggesting a clean install simply an uninstall using avastclear.exe.
Yup I get that Bob, From the comments BWhit mentioned it would be interesting to see if avastclear.exe would have any effect. :-\
I rather doubt that avastclear.exe would be venturing into windows.old, if that is what windows update is finding (unfortunately it doesn’t give much detail).
However, if as the user states, Avast has never been installed, then what is windows update is finding, is the real question.
Hi David, to have some fun, the OP states “currently not installed”. Can we assume that means “never”. 
I would suggest that “currently not installed” would mean not now, but possibly at some time. So there is a possibility it could be hiding somewhere, that however could be difficult to say where as windows update doesn’t give any detailed information.
https://www.tfd.com/currently
https://www.freethesaurus.com/currently
Agreed
So, using the Avast removal tool would be something to try. It can’t hurt.