Build error () with GTK # and Monodevelop on Arch

I am trying to create a GUI for an application I have created, but every time I create a New GTK # 2.0 project and press F5, we get this:

Error CS0103: The Name 'Build' does not exist in the current subject.

      

        
        
        
      

    

It doesn't matter if I drop the Vbox in the window or leave it as new, the error keeps popping up and nothing changes in the MainWindow.cs file.