Inventory Periods - not possibile to close
if you don't find that prod.order in list and not in finished prod orders, could be that someone has tampered with data and maybe deleted the order but forgot to delete related entries.
One way to fix this would be to allow No.serie for Prod.Order numbers to be manual, then create a new Prod.order and enter the number stated in error. If it has some linked entries - they will show up - so you can check lines. If no lines show up try to create one and check the components/routings and so on... .
Leave this order open and try to close the Period again. Should be ok now, then you can just delete the order.
If some lines do show up when you create this prod.order - then it is hard to say what is the best next action. If there is finished qty. Mo e to the order to finished state. If finished qty is 0, then just delete the order.