There is a pre-requisite that require all Mac hardware to be 2013 and later.
The requirement is based on a hardware wifi card that is AC compatible. My mid 2012 MBP retina is just outside of this requirement as it has an older wifi hardware with only N support.
An upgrade to BCM94360CS or BCM943602CS will meet the requirement and allow your MBP to be unlocked with an Apple Watch.
My old card was the BCM94331CS which I will use in the future for my Hackintosh project with my HP Z620.
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