Just wasted way too much time for this:
The Entity Framework provider type 'Npgsql.NpgsqlServices, EntityFramework6.Npgsql' registered in the application config file for the ADO.NET provider with invariant name 'Npgsql' could not be loaded. Make sure that the assembly-qualified name is used and that the assembly is available to the running application. See http://go.microsoft.com/fwlink/?LinkId=260882 for more information.
Obvious perhaps, but not so much anymore while being sure EntityFramework6.Npgsql has been installed... My project used to work a while ago and I could swear I had all the refs intact. After all it would not work otherwise, right...
Anyway, apart from making sure DbProviderFactories and providers are configured, just dbl check EntityFramework6.Npgsql is actually installed / output. If not reinstalling from nuget is a good start. Did work for me ;)