A validated statement was entered with an error on a Party. This can happen because the previous Financial Year was not dealed in Tryton, so there was no Invoice to confirm the choice of the Party. (It could occur too if two Parties had an invoice of the same amount).
I don’t see how to correct the erroneous Party. Cancelling the Statement, then passing it to Draft to correct the Party leads to the loss of all invoices in the Statement.
(I use Tryton 5.0)