Unable to accept iCloud Terms and Conditions in macOS Big Surr

Lots of solutions proposed for this involved signing out and back in with your appleid, but my iMac wouldn't even let me log out!

What worked for me was ticking 'Keychain' in the preferences/appleid/apps on this Mac using iCloud

This produced a message that my account data needed updating to allow this service. There were then a few more security steps involving typing in the unlock code from my phone.

After that the terms and conditions prompt vanished!