Question Multiple Errors on Project Startup

Every time I open a project, there are a lot of errors. Most of them are ".meta file ..... does not have a valid GUID....", error CS0234, and CS0246. This is the first time Unity installed on this PC, so I think there is something wrong with the installation. I tried to reinstall it, but it still has an error. Any solution?

 

Extra unwanted packages in new projects (collab, testing, rider and other junk):

https://forum.unity.com/threads/temp-unityengine-testrunner-dll-error.1133938/#post-7287748

About the fastest way I have found to make a project and avoid all this noise is to create the project, then as soon as you see the files appear, FORCE-STOP (hard-kill) Unity (with the Activity Manager or Task Manager), then go hand-edit the Packages/manifest.json file as outlined in the above post, then reopen Unity.

Sometimes the package system gets borked from all this unnecessary churn and requires the package cache to be cleared:

https://stackoverflow.com/questions/53145919/unity3d-package-cache-errors/69779122