When you haven’t completed any module the option to download your certificate will be greyed out and below that option will be a list of the modules that have not been marked as complete. You can find this option at the very end of the main course page of each of the courses. 

When you complete a module the tick on the right-hand side of the module section on the main course page will turn green.

As you work through your course and complete the modules the list will start getting smaller. 

Once you complete all the modules the list will not be there and the option to download your certificate will become highlighted.