I am using rest api invoicing v2, I obtained token and trying to create invoice draft through API. I am getting the following error response:

"name":"UNPROCESSABLE_ENTITY","message":"The requested action could not be performed, semantically incorrect, or failed business validation.","debug_id":"xxxxxxxx","details":[{"issue":"USER_NOT_FOUND","description":"User is not associated with paypal based on invoicer email."}],"links":[{"href":"https://developer.paypal.com/docs/api/invoicing/#errors","method":"GET"}]}

Can someone help understand this msg?  What does this mean?  Is user the one who sending email or who is receiving? How do I check users?  Should  user who is receiving invoice, be in the paypal systems ? if yes how do I enter or check?   I verified invoicer name/ email and it is correct. 

Thanks.