While there are several potential reasons that you may have received this error message, the majority of the time the reason we are given for this error is "The transaction was declined by the issuing bank."
This may mean that there is a limit on your card's online spending, it may mean that your financial institution has strict criteria for monitoring your account to ensure that you are the one making the purchase, it may mean that there is a hold on your card, or that you do not have sufficient funds, or any other number of reasons.
We recommend that you check your email, or online account, or sometimes an app/text for a message from your financial institution asking if the charge was from you. If you do find that message, it will typically have instructions on how to approve it so that you can attempt to place your order again.
If you cannot find that message, we recommend that you contact your financial institution, make sure all of your personal information and address is up to date and check for any holds, then try again.