Hi Carlos,
We have installed the Simple Certificate on Moodle 3.1. Created the certificate and printed the QR code on the certificate. When I scan the QR code it for a student takes me to https://campus.foodsafetymarket.com/mod/simplecertificate/verify.php?code=7da922b0-44c0-11e8-bfa3-dde0f7acbccc.
I complete the CAPTCHA, click Verify button and noting happens.
Here is a screencast https://www.screencast.com/t/KI4SLH2le4