Q: Why can't I activate iCloud Photo library?

Have you enough free cloud storage?

When I check to activate icloud photo library, in pref system, I have a spinning wheel... and then the check box automatically uncheck!

Does it say "Checking Library"?  Then repair your Photos Library, but make a backup copy before you do that.

See:  https://help.apple.com/photos/mac/1.0/?lang=en#/pht6be18f93

Important: You should back up your photo library before repairing your library.

 

• If Photos is open, choose Photos > Quit Photos.
• Press Option-Command and double-click the Photos icon in the Applications folder (or click the Photos icon in the Dock).The Repair Library window opens.
• Click Repair to rebuild your photo library.

Have you tried signing out of iCloud and then sign back in?

It is a  with the system version AlixBL  is running, Eric. We can't discuss the details here until tomorrow.