This problem is often caused by using a credit/debit card from a country other than the USA to pay for a product/service in the USA. Credit card companies often prevent these kinds of transactions because they suspect fraud. From their perspective, if you always use your credit/debit card to buy products in your country and suddenly, you buy something from the USA, they flag this behavior as unusual. They may prevent this specific charge or they may block your card completely. Usually a phone call to your credit/debit card company or bank will fix this. You simply tell them that you are buying something in the USA and to approve your card for the purchase. This fixes the problem in 90% of cases.
Another common problem is mistyping your credit/debit card number, expiry date or CCV code. Be very careful when you read the digits from your card and transfer them to the online form. It is easy to hit the button next to a number, and then you have a "3" when you meant to type "2" for example.