It probably was the sold out status check that didn’t allow payment for the 2 tickets for “Welcome to Business 2018”. If you go to that event’s page: http://wecm.ca/events/welcome-business-april-2018/
and check how many tickets are left, you’ll note there is only one. So if it had allowed for the 2 tickets to be paid for that would have made for an sell over capacity situation. If they’re OK with getting just one ticket they could pay for just that one ticket if the second ticket gets cancelled.