The course requires you to complete all of the lessons in a section, so if any of them are incomplete, it will prevent you from moving on to the next section.
You can easily check in the course navigator which lessons are incomplete by looking at the small circle in front of the lesson title:
It shows you if a lesson is complete:
Started but incomplete:
Or not started yet:
If a previous lesson is shown as not complete, just click on it and make sure to click the [complete & continue] button at the bottom of the page.
You may also need to finish watching videos as you cannot complete a video lesson without watching all the way to the end.