This error message usually indicates a problem with the aperture assembly in the lens giving wrong information to the camera.
1) Try another lens.
2) Try cleaning the contact points  on the camera and lens using isopropyl alcohol. Remove the battery in the camera. Dip a Q-Tip into the alcohol, remove excess and cover the Q-Tip with a microfibre cloth or similar. Gently clean each contact and allow to dry. Be very careful.
3) If #2 does not work, try to see if the lens has a sticking aperture. Remove the lens and Look into the front of the lens while gently turning the aperture ring counter clockwise and back. The aperture (Iris Diaphragm) should change from a small opening to a large opening, and back, without sticking. If it does stick, Look from the front and from the rear to determine if it is caused by oil, mould or whatever.
4) If there is any crap on the iris, take the lens to a dealer for cleaning, you cannot do it safely yourself.
5) If it is not sticking take the Camera to a dealer for analysis.