The Noun Project processes payments through a trusted, secure service. If your credit card isn't going through, it's because this payment service found an issue with the information that was submitted. Most of the time this happens when the credit card number, CCV or expiration date isn't entered correctly. Other times, it may be because your card is expired.
If you doubled checked your credit information and still receive an error message, please get in touch with your bank. They will be able to clarify why the payment isn't going through.
Additionally, disabling an adblocker, disabling programs that enter your credit card information automatically (ex: 1Password) and switching web browsers may remedy your issue.