That’s fine, just ignore the notification and the ticket(s) will be passed to the next person on the list. 

We'll leave you on the Waiting list for the remaining quantity until those have been allocated to you. Once this has happened, you'll then be completely removed from the list

For example. If you'd requested 3 tickets, were notified about 2  which you didn't purchase then you will be kept on the waiting list for 1 ticket.