You receive the following when running an app with an Entity Framework dependency:
The Entity Framework provider type ‘System.Data.Entity.SqlServer.SqlProviderServices, EntityFramework.SqlServer’ registered in the application config file for the ADO.NET provider with invariant name ‘System.Data.SqlClient’ could not be loaded. Make sure that the assembly-qualified name is used and that the assembly is available to the running application
This issue can occur if you have the correct references to the Entity Framework in your class library, but not in you main executable project in Visual Studio.
In my case it was just a case of using NuGet to install the relevant packages in both the business logic project and also the main executable project.