If you are getting a blank paywall that looks like this:
It's likely because your Payment Settings conflict with your Monetization settings and there is nothing to display, even though the content is still secured.
To fix this, go to . Look over the options here and make sure that either Credit Cards or PayPal are turned on as an accepted payment format and also double check to ensure that the correct currency is turned on as well. For example, if selling in USD you need to make sure that USD is turned on in these settings or else these prices will be hidden in the paywall.
Note: to turn on PayPal you need to agree to the user agreement first.