You can retake the quizzes an unlimited number of times. The purpose of the quizzes is not to trip you up to fail, but rather to act as a yardstick to gauge your progress. If a course is already completed, however, the quizzes will be closed. If you would like to keep access to these quizzes but still finish the Accelerator, all you have to do is pass 80% of the course and your Certificate of Completion will still issue as normal.
If you’re having difficulty with a certain concept, don’t hesitate to ask a question in the Accelerator forum. The guys enjoy providing additional context and welcome any and all questions.