Usually, on a Nikon D80 with an "err" message that won't go away, the Aperture Control Unit must be repaired or replaced. There is a small switch contact held on by a small plastic rivet in the Aperture Control Unit that tells the electronics module in the camera what phase the camera is in during its cocking cycle. When that small plastic rivet breaks, the camera can't determine where it is in the cycle and just stops and displays the "err" message.
There are 2 ways of fixing the problem, replacing the part with the same plastic-riveted switch contact '''or''' taking the Aperture Control Unit apart and replacing the plastic rivet with a small steel screw which can't break again. I've been using the second method for 3 or 4 years now and haven't had a single body experience the same problem again.
Bob Kilbourn
http://www.camerarepairs.com