Log back out of both, then retry using a different browser, or Incognito mode in the same browser. If the item is in your basket, you should be able to go back to the top and change the details - don't forget to uncheck the insurance and 'what cap you want' options, you can always add those later.
If you already have a phone, assuming it is to be delivered to you, is your address in your account details correct, @Anonymous?
Failing that, a call to O2 Sales on 0800 081 0255 tomorrow may be in order. Or message them on Social Media if that works better for you, links to the teams just below. Good luck!