Diagnosis
No customer is assigned to any of the partners involved in thecontract.

Procedure
The partners to be maintained here, only make sense for postings to
customer accounts. You can define which partner is to make the payments
resulting from the condition. First, the partners must be assigned to
the contract and a customer must be created for them.
(This is only possible, if the product type is posted to a customer
account in the company code).