player trigger

If you want to do that for every slide, you don't need any triggers. Just set the Player to use Restricted Navigation.

Here’s how Restricted navigation works:

• Storyline puts the Next button in the Disabled state when the user first visits the slide.
• Storyline changes Next to the Normal state when the slide’s timeline ends. Simply adjust the timeline to adjust when Next will be enabled.
• Next remains in the Normal state whenever the user revisits a completed slide.
• This works even if you don’t show the built-in Menu.
• If you do show the built-in Menu, users will be able to click the Menu to jump to any previously view slide. But they won’t be able to click the Menu to advance to a new slide.

This post has a more info about controlling the Next button, including for slides with interactions: https://community.articulate.com/discussions/articulate-storyline/tip-controlling-the-next-button-101 

If you only want it for one slide, there is no such condition. What you have to do is to start with the Next button disabled or hidden. then change its state to Normal (which enables it and shows it) when the timeline ends.