If you want to use PayPal Direct to bill for Recurring Payments within NopCommerce, then you should:
1) enable the PayPal Direct Payment method.
2) ensure you do not limit the Payment Method to any particular country. (Country Restriction)
3) crucially, and easy to get wrong, you need to setup a PayPal Sandbox Account for a 'Web Payments Pro' Account. No amount of enabling Recurring Payments etc in a normal PayPal Standard Sandbox account will get it to work. You will get an error saying DPRP not enabled for this account..
4) From within the Website Payments Pro Sandbox account, get the API Credentials and put them into the Configuration section under your NopCommerce PayPal Direct Payment Method section. If you forget to do this, you will get another error (I forgot the code).
I recently had this issue of receiving DPRP is disabled error from PayPal. After further investigation, I found out that nopCommerce integrates with PayPal's "legacy" PayPal Pro platform for recurring payments. I had to call PayPal to request them to setup a new account on their legacy platform. The PayPal Pro support people are aware of this situation as other carts also still integrate with their older platform for recurring payments so they should understand your issue, as long as you get the right department at PayPal on the phone.