To clarify, the reason you are getting this error is that you've told our system that you are using PayPal Pro to accept credit cards, which is adds the grey credit card checkout option to your cart as an alternative to paying with a PayPal account (even though PayPal still processes the payment).
PayPal is telling us that you do not have PayPal Pro service. That means that something was missed in setting up your PayPal account with our system, or that you just don't have that level of service with PayPal.
If you never upgraded your PayPal account to Pro and are not paying the extra cost for that then you can solve this problem by going to Seller Admin > Edit Payment Preferences and switching your cart back to PayPal Payments Standard. This does remove the grey credit card checkout option from your cart, but you cannot have that option unless you have a payment gateway to work with it either by upgrading your PayPal account or using a separate service like Authorize.net.