It may be because you are using an outdated Internet Explorer or Java Script. Try updating your browser or using Chrome, Firefox or similar instead. If you are still having issues please email the Web Administrator.
If there is an existing PayPal account associated with the credit card information being entered PayPal won’t allow the card to go through resulting in an error in yellow reading “You have already signed up with this account information”. You must sign into the PayPal account associated with the card to finish the purchase. Or use a different card and pay as a guest.