One of the item on the item fulfillment record cannot be unchecked does anyone know why? They all are lot traced item, they all have been shipped and they all are partially fulfilled from the sales order

Any chance that particular line item has the 'Closed' column checked on the SO?

hmm yes, but all three of these items are on the same sales order and that sales order is closed

you would need to open that line item again in order to make that 'Fulfill' checkbox editable. Marking an order as Closed is meant to prevent further progression of that order/line item.

Aha thanks,didn’t think about it like that @daniel_w