Some sections have a setting that requires you to submit and lock your answer before moving to the next section:
Do not submit and lock your answer until you are certain you have responded completely. Once you lock your answer, you can't edit your response.
If you lock a section accidentally, please contact your instructor and ask them to unlock it. You can send them instructions for doing this: https://help.pivotinteractives.com/en/articles/3452478-working-with-locked-sections