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In Accounts Receivable / Accounts Payable, the documents can be selected using the pick option so all details get picked exactly form the document; however, in the Payee attributes, the document must be selected manually. Ensure that the contract chosen in the Payee attributes matches the contract linked to the selected document and is also the same contract used in the Accounts Payable or Accounts Receivable tab to prevent errors.
For example-
The above steps should resolve the error and allow the posting of the document.
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