Diagnosis
The contract account is not assigned to the business partner.

Notes for the Industry Component Public Services
Postings to a contract account are only permmited for the businesspartner assigned to the contract account as account owner.

Example
Two business partners - A and B - are assigned to contract account VK1.A is assigned to the account with a relationship that is not "accountowner". B is the account owner. In this case, only postings with thecombination business partner = B, contract account = VK1 are permitted.Postings with the combination business partner = A, contract account =VK1 are rejected.