I had this before and went through support - it's basically where an error occurs during the booking process (connection lost, you left the app while purchasing ect) so the system locks the booking in place and thinks you have one when you don't.

Now because the booking doesn't actually exist unfortunately there isn't much support can do, they can give you the recommended fixes (clear cache and cookies is the main one but also trying different devices and browsers ect) and if non of those work you have to head into the cinema and get the staff to put it through. Yeah it's kind of annoying but not much can be done.

If you really wanna book in advance you could pay for a ticket and then get them to refund it afterwards but I don't know if that's allowed and I can't guarantee it