2D Gamekit : `Postprocessing' does not exit in the namespace `UnityEngine.Rendering'

Hi, I encountered this problem too, after reading the source code I found the solution just brought up in the code comments:

// NOTE: If you are getting errors of the sort that say something like:
//     "The type or namespace name `PostProcessing' does not exist in the namespace"
// it is because the Post Processing Stack V2 module has not been installed in your project.
//
// To make the errors go away, you can either:
//   1 - Download PostProcessing V2 and install it into your project
// or
//   2 - Delete the CinemachinePostProcessingV2 folder from your project

And I went to the github page of PostProcessing, found the useful installation wiki, following the instruction in Unity download the necessary part and made it done.

Hope this helps!