In order to use Refersion's PayPal integration, the PayPal Mass Payment API needs to be enabled. You need to contact PayPal to enable this, there is no way to turn this on from the merchant's end.  Below is how you can check if this has been enabled in your account (i.e. if you or someone from your team has contacted PayPal to enable this for you).

First, go to the following PayPal page and sign in, if you have not already: https://www.paypal.com/us/cgi-bin/webscr?cmd=_batch-payment-overview

You will be prompted to select your location, recipients' location, and currency. Choose and click Use Mass Payment :

In the next screen, if you see a message similar to the following, this means that PayPal Mass Payments has NOT been enabled for your account.  You need to contact PayPal directly to enable this for your account, payments made through Refersion will not be processed until you do so.