This happens because the assignment pointed to by the “Continue” button may have been updated, typically by a refresh of the lesson content. Can you try the following and let me know if this helps (the screen capture shows the “Leveraging Pega 7 course, but is applicable to any courses):
Use the Course title link to open the course overview:
Click on the previous lesson that was completed.
Then click con “Next>” until the entire lesson is completed, i.e. you must complete all the quiz questions correctly!
Usually this will then force the “Continue” button to get updated and point to the updated assignment.
Please give this a try and let me know if it works out for you.