When editing a PO line for procurement category you receive Field ‘Company’ and ‘Lot ID’ must be filled in” error. This usually happens when you have brought in the PO line through some form of integration either AIF, excel plugin etc.
This error has also been seen on salesLine and there is a hotfix(KB 2756645) for when it happens on salesline.
There is no record in PurchLineForeignTradeCategory table related to PO line added.
Create a record against the PO line causing this error in the PurchLineFroeignTradeCategory table in your integration and you will see the error disappear.