Paypal Third Party Permission Username

If you try to grant API access to your PayPal account, but PayPal requires third party permission username.

PayPal says, "Enter the name of the user to whom you will grant permissions. If you do not know the third party's PayPal username, contact the third party to request this information."

It means your PayPal account is not PayPal PRO yet.

Simply upgrade your account to PayPal Pro, then you can obtain your API username and password for Cs-cart payment method of PayPal Pro and PayPal Express checkout.