I am trying to pay my subscription but I keep receiving the following error:
Our credit card processor encountered the following error:  Decline. Your credit card has not been charged.
The  Decline code error means that there are insufficient funds on your credit card to process the payment. To fix this issue, submit your payment using a different credit card or call your credit card issuer (the phone number is usually on the back of your card).
For more information about common error codes, see the article: Resolving issues when your credit card fails.