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: