If none of the above helps, see if you can update Visual Studio.
I Had the same issue with Visual Studio 2017 Community when I tried to install Newtonsoft.Json. The ExecutionPolicy
change didn't help (I tried using both PowerShell and Registry Editor). I also tried to uninstall and install NuGet.
After running VS2017 setup file, it asked for update of Visual Studio. All the problems disappeared after the update.