Mac mini: iMessage/iCloud errors after disabling iCloud

From my Mac mini 2020 running MacOS 26.1 (Tahoe)...

I am getting the following messages, and I do not know how to fix this:

'This Mac can’t connect to iMessage because of a problem with “tcarder@capalert.com”.'

'This Mac can’t connect to iCloud because of a problem with “tcarder@capalert.com”.'

Does anyone know what I need to do to stop these messages? I have discontinued what I think are all iCloud features in Settings because I don't want to use iCloud. I suspect I messed something up.

Thank you.