You cannot use the Visual Builder on the protect section itself. This is because Divi requires the custom post to be publicly accessible for this to happen. We cannot do this because we need to keep the post hidden from Google and users on the site.
You can use the classic builder on the protected section. To enable this go to: Divi > theme options > builder > advanced and disable the “Enable The Latest Divi Builder Experience”, then enable “Enable Classic Editor”.
You can use the visual builder when adding the protect module to the page – we have made this module compatible with the visual builder, it is just the hidden content you cannot create using the visual builder.