I had downloaded/important the Post Processing Stack from the asset store and enabled it on the camera, was all good. Later on I had deleted and created another camera, from this moment on this Post Processing Stack just couldn't be enabled. Even when restarting, deleting and reimporting etc. See, nothing there. It appears…
It's probably something simple you just didn't think of, but if you don't figure it out and nothing you try works, export your scene to a Unity package, create a fresh project and import your scene there. If that also doesn't work and something really is broken, delete Post Processing Stack from your project and download…