Flashing " F EE" in the control panel

Turn the aperture ring on the lens to its smallest setting (largest f/number). You control the aperture from the camera body, the same way as on a lens without an aperture ring.

If you need a manual, you can download one from
http://butkus.org/chinon/nikon/nikon_n6006af/nikon_n6006af.htm

This is done in the menu system. Where you change to mode to Close focus. Read the Manual/ If you do not have one www.nikonusa.com

This particular lens is known to be a tight fit, especially when new. Most likely you just need to turn the lens a hair more and a little bit harder than you normally would in order to have it "click" home.

Turn the aperture ring on the lens to its smallest aperture (largest f/number) and lock it. Control the aperture from the camera, just as with other lenses.

Having received no response in over a month, I assume this is no longer a problem.

Set your ISO for auto ISO and see what happens.

Hello,
The 50mm lens is fairly old and uses a different system to the later lenses. On the D90 and other later cameras you should rotate the ring to 22 and lock it there using the lock just above the aperture ring.
To adjust the aperture you use the front dial by the on/of switch.

Try it out in A mode. Rotate the mode dial on the top left of the camera to A (Aperture Priority mode) then lightly press the shutter button to turn the meter on. Then rotate the front dial and you will see the aperture reading changing.

The older Nikons changed their apertures using the ring but the later Nikons use the camera controls.

The FEE message indicates that the lens is not set to smallest aperture (highest number). The camera body expects this so it will have full control of the aperture. It sounds like you may be accidentally bumping or turning the aperture ring on the lens while using it or changing lenses. There is a little slide lock to lock it at f/22 to prevent this. Turn the aperture ring to 22 and move the slider so the white dot is next to the orange mark on the lens barrel.

Take the lens off and put it back on the camera again. Make sure you turn the camera off prior to doing this. Make sure your lens is at it's smallest aperture setting.

could be bent pin in the memory card slot