After watching the materials inside a lesson, a “COMPLETE” button appears at the bottom left of the lesson, when this button is pushed, the following or NEXT lesson will be available.
Close to the end of a course, a second button “FINISH COURSE” could appear on the bottom right side, DO NOT push this button, as it will close the course at the lesson you are busy with. Complete all the lessons to the end.

If this happens, go to courses, click on your active course, scroll down to the bottom of the page, and click on the “Retake Course” button.