If the plugin shows "Recipients: 0", that means that no one has met the criteria to earn a certificate.
Have you added any restrictions to the certificate instance? Go to Edit settings > Restrict access.
If you already have restrictions in place: are you sure that they are set up correctly? If there are zero recipients, then no one has met the criteria, so you may need to analyze the grades, activity completion, etc. that are part of your conditions. Also, sometimes the "must", "must not", "all", "any" options might not be doing what you want. If you post a screenshot of your restrictions, I can tell you what I think they are doing, and see if that matches your expectations. It would also be helpful for you to describe what you want the restrictions to do, too.
And a silly question: are there any Students enrolled in the course?
