Clients still unable to make payments due to "invalid merchant configuration"

When registering for the form, the message "Invalid Configuration" pops up. It says "This transaction cannot be processed due to invalid merchant configuration." I have signed in and continue to have this problem. Also is there a phone number I can call to speak to  tech support directly? 

Have you accepted your PayPal Agreement within your PayPal Seller Account?

Sorry, I meant to say " PayPal Billing Agreement " it's an agreement form that you must agree to within your PayPal Account once they approve you for PayPal Pro. It also enables your advanced features so that they work properly.

I was on the phone with paypal and they told me to delete the API
information and enter it again. I followed these directions and it's still
not working. I don't see an option to accept a billing agreement, but I
believe I already did this when setting up the account. Do you suggest
anything else?
...

The "Invalid Merchant configuration" means that there is something wrong in your Paypal account,still. In light of this you must consult with Paypal support, so they can walk you through the screen in order to accept properly the Paypal Billing agreement.