Nikon D40, Why can't I change the aperature ring?

I have a Nikon D40, I just bought a lens, Nikkor 50mm 1.8D.
The first manual thing is the focus ring, but the ring closest to the body, I can't change it.
I take it off manual and can move it, but then when i go to take a picture, it says i have to put the aperture ring to the minimum aperture (largest/-number)

Why can't I have it on any of the lower numbers?

Being an autofocus SLR, the D40 generally uses the body controls to change aperture, rather than the aperture ring. (Do any of your other lenses have aperture rings?)

For Shutter Priority and any programmed mode, the camera body needs control of the aperture, thus the manual ring has to be set and locked at its minimum.

I don't know if manual selection of aperture is allowed on Aperture Priority or Manual, but I wouldn't be surprised if it wasn't.

Indeed, it is electronically controlled

put it at the smallest f/stop (f/22 for that lens) and push down the little toggle switch at the 2 o'clock position.

Read the manual. You can only use the aperture ring in manual mode. In other modes where the camera uses it's own computer to compute for exposure values, it wants the aperture ring set to smallest size (largest f/value) to be able to control it. You can control aperture size through the body using the wheel (again depending on the specific mode).It's been like that since the old film AF SLR bodies.