TOPIC: freethanks message

When someone who is not registered with our site registers for an event with a coupon code that makes the event free (for our volunteers), the thank you message does not appear as in this link:
https://www.osca.ca/en/component/dtregister/?controller=message&task=freethanks

Where can I go to fix this?

Version? The link you gave will never show anything without having went through a registration.

If a message doesn't display, it's because it's not configured... assuming you aren't using an older version with some old bug or something. There was an issue with free registrations in the 2.7.14 release initially... already fixed and announced. Update to the current build and you should be all set.