iCloud Keychain Won't Sync to High Sierra Mac

I have a few iphones, a bunch of Macs, all on my iCloud account. Some newer, some older.

I just put a new 2009 iMac 10.13 in service. It's an excellent email and web machine.

Except.....

I cannot get the iCloud Keychain and my Safari passwords to sync to it!

I've rebooted countless times. I've logged out of iCloud and logged back in countless times. While logged in to iCloud, I've flicked the checkbox specifically for Keychain on and off. But no matter what I do, I get no passwords.

The Mac is on the latest version of High Sierra with all updates run. It is otherwise an impressively capable machine for something so old. But this one bug is killing me!