Create Ribbon in WPF App with with Visual Studio 2017

You need to add the following Reference: System.Windows.Controls.Ribbon

Then use a RibbonWindow instead of a Window:

<RibbonWindow ...

Also remember to change the base class of the partial:

using System.Windows.Controls.Ribbon;

public partial class MainWindow : RibbonWindow
{
    public MainWindow()
    {
        InitializeComponent();
    }
}