I was trying to get an older package from the a$ to work, that needed post processing... For that, I wanted to install the Post Processing package from Package Manager. After downloading the package, I noticed that various components of that package don't work because the script compile erros from that particular asset prevent the PostProcessing from installing itself properly, although they are 2 totally different systems.
In this case, I needed the Post processing package to properly fix this A$ asset, so I ended up with a Chicken-and-Egg kind of problem.

Repro steps:
- download this smaller package https://oc.unity3d.com/index.php/s/63CCYvuNLpmDZjC or the full .unitypackage from the comments
- open it and notice the compiler error
- open Package Manager and add PostProcessing to your project
- after the loading bar finishes, try to add a Post Processing Volume to the Camera object and notice that the component is not there.
- For this small example, it seems that you can restart the project and the PostProcessing package does end up compiling before you get the compile error again, but for the package you get from OC, this workaround doesn't work.