Why Apple Watch Series 3 won't work with mac OS Sierra — yet
If you've just put the Apple Watch Series 3 on your
wrist and it suddenly won't unlock your Mac, don't panic.
apple Watch and your Mac (2013 and newer) can work together to keep everything on your computer a little more secure thanks to watchOS's Auto Unlock: It lets you automatically unlock your Mac (no passcode needed) if you have your watch on and unlocked.
But if you have an Apple Watch Series 3, you'll need to upgrade your
computer to macOS High Sierra for this feature to work properly —
at least for the time being.
Why? The Series 3 Apple Watch uses the company's newly designed W2 wireless chip; it makes Wi-Fi and Bluetooth faster and more efficient, but requires updated software on the Mac so that the two devices can talk to each other.
Currently, the W2 chip is only supported in macOS High Sierra, which means that any Mac you might want to use with your Apple Watch is going to need the update to function correctly.