What this message means is that the credit card number you entered for payment is associated with an existing Paypal account.  In other words, at some point in time you (or your spouse, child, etc) likely created a Paypal account and added this credit card number to it.  Once a credit card has been added to an account, Paypal will no longer allow that card to be used under a "guest" account.  

To resolve this issue and proceed with the payment, there are 2 options:
  1. Log in to your Paypal account to make the payment instead of trying to pay as a guest.  If you do not know your login info, you can do an account lookup right on the payment page
  2. Use a different credit card for payment, one that does not already reside within an existing Paypal account.