'Newtonsoft' could not be found
Solution Explorer->Right Click on Project Name -> Click on Manage Nuget Packages -> Search for newtonsoft -> Click on install button
Go to Tools -> Library Package Manager -> Package Manager Console
and in the console just write Install-Package Newtonsoft.Json -Version 9.0.1
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