The reasons for such enrollment failure may simply be:
Use of a wrong card number and/or other details
The card limit has been reached, etc.
Some are less obvious reasons such as your fraud prevention and/or other security measures implemented by credit card companies to protect the cardholder against card abuse
What can I do if this happens?
JuanTax has no control over such transaction refusal. Your credit card company would even not disclose the exact reason for the enrollment failure
Give it another try with the same credit card at a later time. In most cases, this would solve the issue.
Try using an alternative credit card if you have one.
Do carefully check that the card details you enter are correct.
Should the transaction continue to fail you might want to get in touch with your credit card company.
