Requirements Viewpoint is a Kitalpha AF Viewpoint.
To see tools and model elements contributed by this viewpoint, you must activate it through the "Viewpoint Manager" view.
To open the "Viewpoint Manager" view, go to "Window">"Show View">"Other..." then open the "Kitalpha" category and select "Viewpoint Manager"
Once the view is open, select an element of your model. Then in the view, right click on the "Capella Requirements" view and click on "Reference".
To check that the viewpoint is working correctly, right click on an Architecture element of your model and check that the Requirements Viewpoint menu is available: