Somehow I can't reply to admin's help on this thread: https://www.opigno.org/en/forum/support/error-when-downloading-certificate
It always said there is a problem with form submission, please wait for 30-50s. After that when I resubmitted, it said "unauthorized to post".
So I create a new topic here:
Thank you very much. That error happened when I download the Opigno 2.10 package and install it without composer. I tried again using the composer installation. It downloaded all necessary libraries. Then the certificate download is working as expected. Thanks again.