Basiclly reiterating what the other commenter said:

If there is less than a certain base amount in stock it actually won't allow you to book it for click and collect as a safeguard against overselling an item (because that 1 in stock could be in a cart or someone might have put it on a completely random shelf in the warehouse or it could have been stolen or missed when ringing at checkout)

Secondly, even if you are first to the store it might not be there. As I said above there is a multitude of reasons why it might have just vanished in thin air (or in a giant warehouse of simular cardboard boxes). If you live close enough to the store, my recommendation would be to go in, see if it's there and if it's not kindly ask an employee if they can check if there is another delivery scheduled. Then I would check the website again the day it is supposed to come in just to make sure the truck didn't get delayed or anything like that.

I'm really sorry it's a bit of a hassle. I wish the system was better and I wish that it didn't advetrize stock that is that low. Considering that desks are in really high demand right now I'd be surprised if you go in and the desk is there.