Error message F-- Nikon D90 w/kit lens 18-105mm

Yes, it has a warranty but not honered in usa

Error message F21? Don't you think it just displays your aperture? (5.6 - 21)

That's the Nikon error message for a lens with aperture ring set to something other than the minimum (i.e. highest number).
The camera insists on controlling the aperture ring, and to do so, the ring must be set to the minimum setting.
Unless it is a "G" lens, which has no aperture ring whatsoever.
The camera body sets the aperture of the lens wide open while auto-focusing, and displaying through the viewfinder. When it comes time to flip the mirror up and take a picture, the camera dials the aperture to the setting you (or the camera) have selected.

Nikon D80 and D90 have that problem especialy with 18 105 kit lenses. You just need to lean the contact between camera body and len. Thry rubing with pencil eraser, but be careful not tu drop any inside of body. Do it upside down.

This is a common problem with D-90's. Nikon won't admit a problem, until they do not much can be done. I have to pull the battery to make my error message go away! I know this info doesn't help much, but your certainly not alone!

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

Try a diff lens on the D90 to see if its the body or the lens. Tony

you must try to remove the lens that lens have some electric contacts and if you set the lens in wrong position that contacts give to the camera some error message

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.

You just found the Nikon D90 F-- error message solution!
I had the same problem with the Nikon d90 F-- error message which drove me nuts!
I called Nikon, they walked me through a lengthy reset procedure which didn't work. After spending a few hours researching online, I found the answer.
It is not the memory card or nor a defective camera body. The lens does not need to be rechipped.
The F-- error means the camera is unable to communicate with the lens and is caused by a poor connection between the ground contact pin on the lens and the metal mounting ring on the camera. Most people buy the camera, install the lens once and then encounter the error. This one time lens installaiton does not give the contact point a chance to seat itself properly. You will find that if you hold down the lens release button and twist the lens as if to remove and reinstall the lens about 10 times that the problem will disappear. You don't even have to actually pull the lens away from the body, just press the release button and twist on and off and allow the friction to provide a better electrical contact.
Too bad Nikon doesn't tell their customers about this simple fix. Other than that, great camera. Good luck.