Can’t Enable Apple Watch Auto-Unlock
Are you seeing the "Allow your Apple Watch to unlock your Mac" checkbox?

You may need to enable Continuity.  Make sure both devices are on the same WiFi network, and Bluetooth is enabled.  The article also had an error, it says 


  "[...] make sure that Disable Automatic Login is unchecked. If the
  option is grayed out, you might need to click the padlock icon at the
  bottom of the window to make the necessary changes."


I think they meant that the box is checked.

EDIT: Here's the Apple article you want to reference: https://support.apple.com/en-ca/HT206995