When a Steam purchase fails to go through, it is likely due to a problem with your selected payment method. The most common causes of a failed purchase include insufficient funds, an incorrect billing address, or an outdated card expiration date. Additionally, some banks may block purchases made through Steam for security reasons. To resolve this issue, you should first check that you have enough funds in your account and that the billing address and card expiration date are up to date. If the problem persists, you may want to contact your bank to determine if any restrictions are in place to block Steam purchases.