Account Sharing feature available to Dev Accounts?

I am trying to send agreements on behalf of another user using REST API V6 (Transient Document) 
I followed this tutorial:

https://helpx.adobe.com/sign/kb/how-to-send-on-behalf-of-another-user-using-rest-api-transient-document-adobe-sign.html

But I always get back a permission error:

{"code":"PERMISSION_DENIED","message":"User provided in x-on-behalf-of-user header does not have required permission to perform this operation."}

The x-on-behalf-of-user  email I am using is part of a group and account,
not sure what am I missing, can you share an example with the needed scopes?