Unable to unlock Mac as its locked with Find My

Hi,

If your Mac is locked with Find My and you can't unlock it, you'll need to use iCloud.com/find to unlock it remotely, or take it to your local retail AppleStore, Genius Bar or Apple Authorized Service Provider if you can't remember the passcode. You can also try to remember the passcode you set when you locked it with Find My. 

Genius Bar Reservation and Apple Support Options - Apple

Find Locations

Were you the one who locked the Mac? If not, how did it come to be locked? Did you buy it second-hand?