Package manager related compilation issues, how to fix?

Hello, I'm having some issues with what I believe is a problem with the package manager. It's showing some errors related to PopupField.cs (part of a built in package with unity). This is with 2018.3.6f1

What I've tried to do to fix this so far:

- Original Idea was to open package manager and disable and re-enable the packages in use, as shown on screenshot package manager is not to be found.

- Help -> Reset Packages to Defaults

- Reimport all

- Delete library and temp folders of project

- Delete unity, all unity folders in AppData and Program files and reinstall

Nothing I've tried so far has worked, are there any other steps I can try?

 

Me three.

 

I've had to delete it from a couple places on a couple coworkers' computers. On Windows, I deleted Library\PackageCache in your project, AppData\Local\Unity\packages, and AppData\Roaming\Unity\Packages and it's fixed it in all three (maybe 4?) instances.

One time, though, what looked like a package compilation issue turned out to be a problem with our project failing to compile the managed assembly. For some reason, though, it didn't show up in the console so I'd check the editor log for compilation errors if my first suggestion doesn't fix it.

 

Thanks for your help all! I deleted the manifest.json file (same as what karl suggested) which seemed to sort this out. Thanks again