If re-entering your credit card doesn't solve the problem, there are a few common avoidable errors to consider:
Suspicious charge - Your bank might be suspicious of an unauthorized charge and therefore block the payment. If this happens to you, you can lift the block by calling your bank.
Use a different card - If you have this possibility, change your payment information and try again.
Contact us - If all else fails, contact us at firstname.lastname@example.org and we will get in touch with our payment system provider and find out the reason of payment failure.