There are multiple reasons for this to happen. Looks like your account was flagged because either you bought too many Cartel Coins or there was a problem with your credit card. Unfortunately, only Customer Service can help you with that, and it is their policy to only fix account issues via phone, so there's no point trying to send them an e-mail or in-game ticket.
In the worst case, you can always buy a physical Cartel Coin card at a retail store, or a digital code online. Then, the purchase should work without a problem and you can just redeem it on swtor.com to get the coins. The best part is that you will receive a pet along with it for free. Here's a list of the various stores that sell Cartel Coin cards: http://dulfy.net/2015/05/28/juvenile-rancor-pets/
