Turn off all iCloud services and try again.

That is an unacceptable blocker - need good CC info in order to disable iCloud? C'mon.

Now, I get the same "Turn off all iCloud services and try again." message - the system already asked and disabled "find my Mac" and my CC is good. This is an incomplete answer. Where does one see the "iCloud services" so that one could "turn them off" ??

Have you enabled 'Find My Mac/iPhone/iPad'? You will have to disable it before you can sign out.

I've not used Keychain with iCloud so I can't answer that. Doubtless someone else can but you might want to start a new thread and entitle it 'Cannot turn off Keychain in iCloud in order to sign out' so that the problem is clear.

I ran into the same problem. Then realized that my credit card on file had expired, so went in and updated that in account details, and then it let me sign out.

Try System Preferences/iCloud and uncheck the boxes.