When there is low inventory it's not unusual for a cabin to appear to be available but as you click through the booking process it's no longer available. This effect is more common on unique cabins where there is only one or two on the ship. Imagine dozens or hundreds of people all vying for that one cabin at the same time.
Someone else clicking on it and window shopping can temporarily take it out of inventory. This is done so multiple people who are going through the booking process can't all book it and they end up with several bookings for the same cabin. If that or those other people back out and don't actually book it then one lucky person can click through a little further in the process but in so doing they once again take it out of inventory for everyone else.
In some cases a travel agent can help cut through the clutter and actually book it and in some cases they won't be able to book it either.
@Yo2slick Good for you, Hope cruising is back to normal soon, We have a back to back booked in October on Rhapsody, in the Royal Suite hope it still goes on?.