I know this is a common problem, and I do not know why this is happening or how to fix it. I have Unity 2018.3.0f2 and When I install Post Processing stack via the Package Manager for version 2.1.2, the behaviour script is missing. I can create a profile and play with the stack, but I cannot attach the component to the camera. It does not appear in the list. Does anyone know how to fix this issue?