Mind you, Square has been having all manner of problems recently with both PlayOnline and FFXI, with attacks by hackers causing server outages for extended periods, so whether this is just another example of this I am unsure. But, it’s possible.
Otherwise, is your credit/debit card active? If your card’s credit is maxed out or doesn’t have funds on it (if it’s a debit card), or is not a Visa or Mastercard brand, it is not accepted by PlayOnline, although I thought it gave a different error message to the one you got if that is the case. Try another Visa or Mastercard if possible, and see if that works (I’ce heard people have had problems getting PlayOnline to accept certain types of Visa Electron cards as payment, so this could be another cause.