1) That message might mean
a) That they added it in there IN GENERAL since some part of users are on Windows 8.
b) There's a mistake and it thinks you are on Windows 8.
2) It seems like your credit card got declined and so you might have to call your credit card company and have it re-authorized.
That's the only thing I can think of. If it's not that then it's perhaps a bug on B&N's end. Restarting the Nook and trying again would help. As would just buying it from the website.
Check the credit card via the My Account page on the website. Is the card expiring soon? Expired? Maybe try deleting the card (since it isn't working anyway) and reentering it?
I have seen the Windows 8 error, too, but putting a card into the website seemed to work, at least in that one instance I saw.