Error CS0246

It feels like Newtonsoft isn’t installed in your package mangler. Check it in there or…

If you want to avoid the Package Mangler’s UI, just add this to your Packages/manifest.json file:

"com.unity.nuget.newtonsoft-json": "3.0.2",

Beyond that, perhaps a reimport-all?

Also, try this to reduce mess in your project:

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

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