No Entity Framework provider found for the ADO.NET provider with invariant name 'System.Data.SqlClient'. Make sure the provider is registered in the 'entityFramework' section of the application config file.

I just had this error when calling a library from a console app. The library has Entity Framework 6 installed. It turned out the EntityFramework.SqlServer.dll is not copied to the console app's bin folder. You can either manually copy that dll to the bin folder or simply install EF 6 in the console apps.