Variables & triggers

The problem with this trigger is the "when." It's set to run when the timeline starts. Well, that moment has come and gone before the user has a chance to click anything.

The trigger below works. But you're not seeing it because it fires before the trigger that shows the layer for the last-clicked/Visited icon. (You can see this for yourself if you change the Slide Layer Properties so they don't hide other layers.) 

In any case, if the final layer did show as soon as the buttons are Visited, that would prevent the user from viewing the layer for the last-clicked button. 

This post has a demo file that shows how to ensure users visit all layers before the Next button is available. You're not using the built-in Next button, but the same programming concept applies. TIP: Controlling the NEXT Button 101 | Articulate - Community