I had gotten error 601 when trying to enter my information via the Mog Station. My solution was to go to the Square Enix Account Management page and update my payment method there before returning to the Mog Station and proceeding with payment.

My girlfriend, on the other hand, does not even have the option to update her payment information on the Account Management page. That entire section is missing from the page for her. The only option she is able to currently see/use is Crysta. Although how she'd pay for the Crysta is beyond me as she can't enter a credit card.