'Newtonsoft' could not be found

Go to Tools -> Library Package Manager -> Package Manager Console

and in the console just write Install-Package Newtonsoft.Json -Version 9.0.1

Solution Explorer->Right Click on Project Name -> Click on Manage Nuget Packages -> Search for newtonsoft -> Click on install button

You have to get the correct NuGet package. From your link:

You can open the NuGet package manager in: Tools -> Nuget Packet Manager -> Manage Nuget Packages for Solution..

For me I had the package already installed but it was missing the references and it could not be added. Helped me: in NuGet Package Manager to downgrade and upgrade Newtonsoft.Json