To award users who complete the story with the required pass mark and within the required time with a certificate, go to the Story Settings under Limits & Certificates. Set the "Award Completion Certificates" option to "Yes"
Below are samples of what the completion certificates will look like. To ensure that certificates can't be edited/reproduced, a unique QR code is generated for each certificate. When the code is scanned, it will take you to the original certificate stored on the Falkor servers.