Factory Reset
MacBook Air, OS X El Capitan (10.11.6)
MacBook Air, OS X El Capitan (10.11.6)
That Mac has been protected with a frimware password How to set a firmware password on your Mac - Apple Support
If you don't remember your firmware password you'll have to take it in to the Apple Store or AASP along with your original purchase reciept to have it unlocked.
Have you followed the instructions here?
Though may be looking for your Filevault password.
Factory Reset