Unable to sign out of my iCloud account due to an unknown error

If you can't sign out of your iCloud, it's likely because you have Screen Time restrictions enabled on your device, which can prevent signing out unless you turn it off first; check your settings to disable Screen Time and then try signing out again. 

Might also be caused by a VPN that is active. If you are using a VPN, try turning it off and give it another shot.