On shutter priority mode and the Fstop won’t change and constantly getting dark pictures

Your lens' widest opening is probably f/1.8. It can't open any wider to let more light in. It's already as wide open as it can go.

If you have selected an exposure time ("shutter speed") that is too short to properly expose with f/1.8 and your selected ISO setting, then you need to either:

• Increase the ISO setting and be prepared to deal with the reduced image quality
• Increase the exposure time (reduce your "shutter speed")
• Add more light to the scene
• Use a lens with a wider maximum aperture, though that isn't very practical when you're already at f/1.8 because f/1.4, which is the widest many common lenses will go, is only two-thirds of one stop wider. There are very few lenses in the world, at any price, that are wider than f/1, which is less than two stops brighter than f/1.8.

Those are your only options.

Sounds like you have a ƒ/1.8 maximum aperture on the lens. The camera would like to make the exposure brighter, but that's as wide as the lens's aperture will go.

The problem is the scene you are shooting isn't bright enough for your shutter speed and ISO settings. You need to slow down the shutter speed, or dial up the ISO, or both.