XML : App.Config transformation not working

I am trying to change the app.config dinamically on my build server, I created a Configuration called Build, and I have these 4 .config files.

this is my app.config

and on my app.build.config

However when I created the build definition and added the configuration =Build I get this error

  rror: System.Data.SqlClient.SqlException: A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: Named Pipes Provider, error: 40 - Could not open a connection to SQL Server) ---> System.ComponentModel.Win32Exception: The network path was not found    

Which means is taking the app.config file and not the transformation