"This transaction cannot be accepted" error


We have an error when customers pressing the submit button after entering charge card payment: “This transaction cannot be accepted.” Authorize.net support told me to check out API records, but I don’t know where they are. They also suggested to create a new transaction key, however I do not know where to add this key in our cPanel


a transaction key is for their api, not cpanel.

Is this part of a shopping cart system?