For PayPal Pro, you only need to enter the API User Name, API Password and API Signature.
the Merchant ID setting you are referring to is only needed if you select the Option Enable 3D Secure, this is a completely different service not provided by paypal. dont enable that option or enter anything in the 3d secure section unless you have an account with them
in our learning resources section, we have a tutorial for "Signing up with the PayPal payment gateway":
I am going to start a private support incident so that we can look into the issues you are having on going to paypal.
To view and edit your support ticket, please log into your support history: