For a quiz to be mandatory, you need to set it up as a prerequisite for the next slide. You cannot complete the course if you haven't seen all the slides and you cannot see the slide until you have passed the course. 

To set this up:

In the Content Explorer for the course, on the slide after the quiz, right-click and choose Manage Prerequisites.

In the Prerequisite Editor, under Requirements, click Add page. 

On the Page selector popup, choose a page and then click Insert. 

Enter the error message that you want to appear if they have not managed to pass the quiz. Tick the check mark to save the error message. 

On the quiz itself, you have to choose what constitutes as a pass. You do this on the asset by choosing the mastery score, for example, 70%.