Could you please try to set the version for the package manager package in your project’s manifest file to ‘2.0.3’ and of the HDRP package to ‘4.1.0’?
This thread might be relevant to you as well: Script compilation issues upon upgrading to 2018.3.0b2
The post processing error is fixed in the latest version of post processing stack v1 on Github. GitHub - Unity-Technologies/PostProcessing: Post Processing Stack
However, since 2018.1 it is no longer officially supported and we’d advise you to switch to v2, available via the package manager.