Site Administrators can turn on the Custom Certificate and create templates etc from the Site Administration > plugins > manage activities
OR Custom Certificate settings.
Usually editing teachers have the permission
to add and customize the custom certificates within their course.
Non-editing teachers usually do not have this permission set so they would be unable to add or update.
You could check the permissions by going to:
Site administration > users> permissions > define roles > select the role to check > search for certificate ... check or change permissions as needed.