Course completion certificates can be downloaded via a button that appears at the top of the related course page and in the table displayed on your certificates page.
Usually any issue with access to a course certificate is because the related course progress is less than 100% which means that not all required course activities have been completed. Click the My progress bar at the top of the course page to find out which required activities have yet to be completed. Click on the activities in the drop down progress list to scroll to the related place in the course. See this question for more information.
Occasionally the certificate download button is displayed, however on clicking this button there is a problem accessing the certificate. This is usually a browser or device incompatibility issue and the best solution is to try a different device or browser. We recommend a laptop and Google Chrome to avoid compatibility issues.