Why can't I buy a Nook Miles Ticket?

I'm curious, why am I unable to purchase a Nook Miles Ticket? I've been saving up my Nook Miles and have more than enough to make the purchase, but the option to buy one seems to be unavailable. Is there a specific reason behind this, or is it a temporary issue that will be resolved soon? I'm eager to use my Miles to travel around the Animal Crossing: New Horizons world and explore new areas, so any insight you can provide would be greatly appreciated.