Using REST API v6, receiving code":"BAD_REQUEST","message":"Recipient set is not enabled"}

Background:

Using REST API v6

Authentication style: Integration Key

Users are created and assigned one specific custom primary group to provide differing branding experience.

Issue:

Response to the POST at https://api.na4.echosign.com/api/rest/v6/agreements

Status: 400 Response: {"code":"BAD_REQUEST","message":"Recipient set is not enabled"}

Issue to Note:

1) Creating a new user with only the default Group membership, can post agreements... but does not have the branding details that we need.

2) Researching the error message only turns up supports inquires from 2017, I believe that the term 

Support Question:

1) How do we "enable" a "Recipient set"?

2) Why is this only occurring on accounts when a custom group is set as the primary group.

Thanks for the help

Ed