Buying iCloud+ storage

I’ve bought an iPhone 14 and want to give my 13 to my partner, who is still running an iPhone 6. He can’t back up his data to the free 5GB so has attempted to buy 50GB at £0.79 per month. We seem to be stuck in a loop of clicking the button to buy, getting a message that the card has been verified at the bank then the same screen shows with the button gone and the pinwheel rotating. We have left it plugged in to the mains for 24 hours and nothing changes. No charge has appeared on the bank account.

Any ideas what’s going on here? Help please