My card was declined but there is nothing wrong with my card.
This type of problem is often caused by using a credit/debit card from a country other than the USA to pay for a product/service being sold from the USA. Credit card companies often prevent these kinds of transactions due to suspected fraud. 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. Contact them and explain your purchase.
Another common problem is mistyping your credit/debit card number, expiration date, or CCV code. Be very careful to type correctly the digits from your card into the online form.