My Layers are not Showing in Preview?

The second layer, navigation in your case, does not show unless something on the first layer, introduction, has a trigger to show it. I think you are misunderstanding the purpose of layers in Storyline. Perhaps you're thinking of layers in Photoshop or Flash. This is not the same thing. 

You need to put your Previous and Next buttons on your main timeline with the text box that says Practice Course. 

Layers would be used if, for instance, you had a flipbook. The first page of the flipbook would be on your main timeline (your introduction) then each of the other pages of the flipbook would be on subsequent layers. The subsequent layers would then only show when something on the main layer was triggered to show them.