Cannot complete an iPhone reservation despite it being in stock.

I’m trying to reserve an iPhone X to upgrade to from my 7+ on the iPhone Upgrade Programme, I go through the reservation process at 6AM when the stock is released then when I get to the final stage of the reservation it says their having trouble processing my request despite me there still being stock in the store (I can see because there’s still iPhone Xs to reserve)

If anyone knows how I can resolve this issue or reserve an iPhone X another way that would be great. Thank you