Help Needed: Storyline Build with Video Completion Logic (6 Videos)

There are different ways to accomplish this. Here's one:

• The button to show video 1 has an Initial state of Normal. The other buttons have an Initial state of Disabled. That forces the user to watch video 1 first.
• Each button shows a layer with the video.  
• Each layer would have a button that hides the layer. That button shouldn't appear until after the video completes. Add a condition to ensure this trigger doesn't change visited buttons back to Normal if the user watches the video again. For example, base it on a T/F variable that is changed when the video completes.
• On layer 1, include triggers that change the state of the other buttons to Normal when the media (that is, the video) completes. 
• If you set the layers to present as Dialog layers, the user won't be able to interact with anything except what's on the layer. And if they can't close the layer until the video completes, you could  show the Congrats message on the base (and enable the Next button) based on the state of all the buttons being Visited. 
• Alternately, use T/F variables to track that the layers are completed.

You might find this post helpful. It's about controlling the NEXT button, but the same programming works to control when another object shows: TIP: Controlling the NEXT Button 101 | Articulate - Community 

Here's more info about Dialog layers: https://www.articulatesupport.com/article/Storyline-360-Dialog-Layers 

And this shows how to set an Initial state: