Is there a way to have someone retake a course that is assigned to them until their score is 100%?

Navigation: Admin > Learning > Courses

Yes, that is possible. The course author can set the retake attempts while creating a quiz unit. 

1. Toggle the button to Yes to allow users to retake a failed quiz.

2. Set the number of retake attempts. For instance, a user can retake the quiz 10 times to try and score 100 percent on it. Once a user exhausts all the retakes, they can not take any more attempts until the number of retake attempts is set higher by the course author.