How to resolve 'Sign in to your Apple Account' error when sharing albums in Photos on iMac?

I have just got a new iMac, macOS 15.6. When I try to share an album in Photos (ver 10.0), I get a message saying "Sign in to your Apple Account", but as far as I can see I am already signed in. When I go to Settings and click on "Apple Account" I see my username, Personal Information, Sign-In & Security, Payment & Shipping and at the bottom of that window a Sign Out button, so I guess I'm signed in?

Any ideas?

[Re-Titled by Moderator]

Original Title: Photos on iMac