Nikon d3200 (aperture scale unable to change)
You haven't given much information to go on. There are many possibilities as to what may be preventing you from changing the lens' aperture.
Depending on what Exposure Mode you've selected, the aperture setting is adjusted by rotating the command wheel on your camera. If you are in S or M shooting mode you also need to press the +/- button while moving the command wheel to adjust the aperture. Moving just the wheel will change the shutter speed in S an M exposure modes.
If the lens is already set at the widest aperture, you won't be able to open it any wider (lower f-number). If the lens is already set at the narrowest aperture (highest f-number), you won't be able to close it any narrower.
If you are using an older Nikon "D" series lens with a manual aperture ring on the lens, you need to be sure the manual aperture ring is set all the way to the highest f-number (narrowest aperture setting) and locked in there or your camera can not control the aperture of the lens. If you are using a non-CPU lens then you will need to set the aperture manually on the lens, but the through the lens metering will not be accurate and you will need to set ISO and Shutter speed manually.
If the aperture value will not move in only one direction, but will move in the other, then you are probably bumping up against a limit you have imposed using another setting, such as minimum shutter speed. If the camera is flashing the shutter speed on the info screen or in the viewfinder it is telling you any further change in aperture in that direction will result in the need for a slower shutter speed than has been set by you for minimum shutter speed. In that case you need to disable the option that limits the minimum shutter speed (or also enable an option that allows for raising the ISO automatically to compensate for the narrower aperture).
Do you have a flash enabled and are you are trying to open up the aperture to a setting that would require the shutter speed to be shorter in duration than the camera's flash sync speed? If the shutter speed is flashing on the info screen or in the viewfinder and it is the camera's sync speed then you need to reduce the ISO setting to allow for a wider aperture with the sync speed. If you are already at minimum ISO, then a neutral density filter would be the next option.