Do not know if you have solved your problem yet or not, but am currently going through this issue myself. From what I understand at this moment from the other posts I have read, you must contact their tech team via https://www.paypal-techsupport.com/app/ask and specifically asked for the DPRP feature to be turned on... I am still waiting on their reply so I cannot promise that this will work.
See DPRP is disabled for this merchant sandbox account
UPDATE
You must email them to activate it. Didn't find out if this is just for older accounts or for all accounts, but was told it may take 24 hours for the setting to take affect. Had to jump through some hoops to talk to the right people on the phone, but got it done.
Oh! And was told best to regenerate the app keys once it is on.