Someone else probably booked the last tickets at the same time.
Please try to remove the cookies and the cache from your browser. To do this press the CTRL key and F5 at the same time. These are both caches, that can cause outdated ads to appear. If you reload the website the event should then be shown correctly in its updated form.
Otherwise you can try and open the website in another browser or in a private browsing window and search for the event there again.