It can always be the case, that the event is sold out when you want to purchase your tickets. Another option can be that the event is being processed by our system, because we e.g. are adding more seats. In such cases tickets can't be ordered at that moment.
Please check again later to see if the tickets are available again. If the tickets aren't available for a longer period of time, the event is probably sold out, or under system adjustment maintenance. If you should still have questions about a certain event, feel free to contact our customer service.