The hardware and OS versions involved: Mac: 2016 12" MacBook, Sierra 10.12.5 Watch: Series 2 SS 42mm, wOS 3.2.2 iPhone (no idea if relevant): 6S, iOS 10.3.1 What I'm encountering is as follows: I go into Security and Privacy, and there's the option to check "Allow Apple Watch to unlock your Mac." It's enabled, and I click the check box. After a brief delay, there's a message next to that saying "Turning on..." with the spinning ball. A second or two later, I'm redirected to the iCloud system preference. In a couple cases, I was prompted for my iCloud password to access the iCloud Keychain. However, when I type the password, there's no other reaction. It's just the iCloud system preference. In the other cases, I get the iCloud system preference and nothing more. When I return to Security and Privacy, the checkbox isn't filled, and when I click the check box, it's back to the top of this list. I've already enabled 2FA, and I know that as far as hardware and software are concerned on the base level, everything is eligible.