Question: AirDrop not working MAC OS Sierra 10.12.6
I've recently experienced the same problem and discovered why:
I recently installed ExpressVPN using a station about 100 miles away. This VPN blocks AirDrop for the Desktop MAC. Turn off the VPN (on both devices) and voila! It works...........
iMac (21.5-inch, Late 2013), OS X El Capitan (10.11.4)