Most payments failures are because they were not approved by your bank. For more information on why please contact your bank or credit card provider.
If you see the payment pending on your credit or debit card account, then it may have been rejected by the payment processor due to the anti-fraud billing address or card verification checks. This indicates that either the billing address that you entered on the form did not match the billing address you have set with your card company or that the short 3 or 4 digit card verification number (CVV or CVN) that you entered on the form from the back of your card does not match what the credit card company has for your active card.
In these cases, the payments are just in a pending state and have not been received by edX. Since the processor has not completed the transaction the funds will automatically return to your account within a few days. You are welcome to verify your information try to pay again.
If you are receiving a receipt error for a PayPal order and the course does not show verified on your edX dashboard, then likely there is an issue with your PayPal funding source. Please contact PayPal support for more information.