I was testing the certificate module with Moodle 2.8, and it worked for me. Make sure you are using the latest version. (I got the latest code from git.)
Just to let you know that I did some re-work on this plugin and released it as a block. I have shared it to the community and hopefully someone will find it useful. Here's the link to the plugin - https://moodle.org/plugins/view/block_report_certificates
Any issues or feedback, please let me know.
M