As I said yesterday in comments plugin, the block does not work in version 2.8.6+.
I enclose two images with errors.
I hope he can solve easily.
Greetings and thanks.
P.D: ignore a line " WHERE cm.module = 4 AND ", is repeated in the 2 images.
I have just updated the codes and tested on my moodle 2.8.3. It works fine now. Had to change the course module and date format. For some reason I had to uninstall my previous install and reinstall the plugin again. I haven't submitted the final code here but wanted to check if it would work for both of you.
Please download the zip from here - https://github.com/manieer/moodle-block_report_certificates/tree/For-Moodle-2.8
You will need to do a manual install of the plugin. Also this plugin is really aimed for the 'My home page' where you can place it on the main content pane.
Please let me know if you encounter any problems.