Learners Getting Error While During Course

I’d suggesting checking the following:

If you’re using an LMS, then in the settings for your table of contents (Themes \ Table of Contents à Settings button in the Skin Editor window [PC – not sure if Mac is the same]), ensure the Self-Paced Learning checkbox is cleared. This setting causes the project to use cookies for suspend/resume to track progress and store variables and their values. You won’t want that because you’re already reporting these to the LMS.

If the above has no effect on the issue, check your quiz settings (Edit \ Preferences à Quiz à Settings category). It may help to change the Required dropdown to “Optional – The user can skip this quiz.” If you still require your learners to answer every question, you can then clear the checkbox next to the Skip setting in the same window and, in each quiz slide’s Quiz properties tab, under the “Captions:” heading place a check in the box next to “Incomplete.” This will display a message advising the learner they have to complete the question in order to continue.

Did you set up bookmarking by the LMS?  Once a question has been answered within one quiz attempt, you cannot change the answer anymore. How many quiz attempts do you have?