There may be a variety of reasons why a credit card payment is not processed.
In-App Payments:
Keep in mind that the system only allows to process credit card payments through Clearent with IPs within the United States. External IPs are not allowed.
Make sure that your company'scurrency and your customer's currency is USD.
If your company is multilocation, make sure you enable the integration for each location.
Terminal Payments:
Payment failed. There was an error connecting to the terminal: check your connection and try again.