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).
5) Tick the box to say Use Sandbox' and thats it.
Finally, it should then work perfectly.
Let me know if these instructions work for you...