A quick primer on how SE's billing system works:

If your card is declined or otherwise fails to process for any reason, a 24 hour block is made on your account for billing. No matter how correct your card details are, attempting to re-register your card will fail. To add insult to injury, every time you try, this timer will be reset back to 24 hours.

This means that you need to wait over 24 hours before you try again if you want it to succeed. It's dumb, yes, but it's worse that it doesn't explain this anywhere.

Last time my account got locked like this, it was because my address had changed. Had to update my address from the Square Enix account site, as that's what it sends to your bank when verifying your card. Dozens of attempts to register my card failed, but after reading about this little 24 hour lockout nugget somewhere, I waited two days, just to be sure, and bam worked first time.

1) Make sure your card/bank/address details are definitely correct.
2) Wait over 24 hours before trying again.

If you can't wait that long for whatever reason, you can still apply game time to your account using a time card, I believe.