Catalina iCloud log in errors

I have a similar issue on my MacAir with email and icloud. It complains that the account is already there and wants me to remove it from the internet accounts but its not there. I've tried logging off all of my devices from icloud to no avail. I think somehow it's tied to me changing my apple ID. It was with a defunct email that didn't exist anymore and after the upgrade it was trying to send me email even though I had a different email attached to it (an icloud email). Somehow in the process of the upgrade I was allowed to change my apple ID so I figured it was ok to use the icloud.com email for my ID since it was already tied to the icloud email and icloud, etc. That may have been my problem but if so it's weird because the iphone and the ipad are fine with the change and I can still see my icloud.com emails on those devices. To add to the weirdness the email app on the MacAir allows me to go through the drill of adding the icloud mail account but does nothing with the data I give it. It acts like it takes it but no mail box appears. I'm thinking of reinstalling the upgrade.

I finally fixed it last night. Initially I tried reinstalling Catalina but that messed up my APFS and I got the "Could not create a Preboot Volume for APFS Install. I researched that and apparently somewhere around Sierra Apple changed from HFS+ to APFS. Everything I found was saying I should delete the hard drive but I wasn't comfortable doing that. Figured that by now Apple had figured out how to roll that into the upgrade install so I opted for fixing the volumes on the drive. Then I reinstalled the OS. After that completed I hoped my issue with my id would be resolved but it wasn't. It was still complaining about the account I was using (my appleid) was already active. I stumbled upon someone (I'm going to go back and try to find their handle to give them credit) that said you should :

0pen the Terminal app

Type defaults delete MobileMeAccounts

Hit enter to execute this command. This will force macOS to sign you out of iCloud - now you should be able to log in from the iCloud pane in System Preferences.   

When I did this I got a message that the default shell had been changed and it prompted me to change it to the "zsh shell" which I did and then proceeded with the above.  The prompt came back and said that a MobileMeAccount didn't exist. Regardless, it did FIX THE PROBLEM!

Hope it works for you.