That's the reason you CANNOT use S-mode. With S mode, you set the shutter speed and the camera sets the aperture - which of course it cannot.Try making pictures in S-mode. Camera can't change aperture (you need to do it manually on the lens).
In A mode, you set the aperture and the camera sets the shutter speed.In A-mode you can't adjust your shutter speed
In M-mode, you can't adjust your exposure compensation
But didn't know that you can get metering probs when in S-mode.