I've tried everything to fix this. Restarting my computer, closing Blender and opening it again, uninstalling Blender and installing it again. Unless it's a bug, I don't know how to fix this. Advice anyone?
Also here's a screenshot of the issue I'm having.:

I made the mountain after selecting "Spherical Falloff", I'm trying to make a hill, and it's obvious that is no hill.