There may be several reasons.
Most likely, the bank which issued your Credit Card blocked it for international payments or maybe all online payments. Contact your bank or whoever issued the Credit Card to you and requested to unblock the Card, so it will be allowed to do online payments and international payments.
You might have made a mistake and write incorrect billing address or other information. Check again and make sure your address is entirely correct.
You may also try to use a different Credit Card or even different payment method.
If you made sure your billing address is correct, and your Credit Card is not blocked for online and international payments, and you still have a problem to pay us. Please contact us. Remember, never write full Card number in any written communication.