iCloud won't accept my password to turn on "sync this mac" for Photos

(iMac 24, 2023, Sequoya 15.3.1) I turned off Photos sync in iCloud+ to create a new, smaller photo library. Now I wish to turn Photos back on to sync to iCloud. My Apple and Mac passwords don't work to turn it back on.

A password entry window appears with the statement "iCloud Preferences wants to make changes to your account. Type your password to allow this." The account user name is shadowed out, it shows my first name only. Not my Mac user name. Not my apple account user name. Just my first name. NO password works - not my valid, current, working apple account password nor my Mac password. And I can't change the user name in this password entry screen. It doesn't accept fingerprint. It doesn't signal my watch to double-click to enter my password.

I checked my iCloud+ account, everything is current and all other items are syncing to iCloud. I don't see my first name only used as my user name anywhere in my Apple accounts.