I downloaded and installed Unity 2018.2 on my Windows 10 machine today. It seemed to proceed without any issues. I then opened a previous project, double click on a script in the editor to open in the Visual Studio 2017, and simply try to build the project. I made no changes. I get 140+ errors stating that scripts relating to the Unity Package Manager, Text Mesh Pro and Standard Events can not be found. Basically any package located in the “Packages” folder cannot find scripts. The editor can still get into play mode, but Visual Studio will not allow me to build the project or enter debug mode.
I have attached a few screenshots that show the errors in more detail