This field cannot be updated, as there are Open Sales Order Lines
User usually encounter this error if they try to deactivate an item and there exists open order lines/deliveries for the same item. And this type of error message are easy to handle and debug, and possible solutions are to close/cancel the order lines and corresponding delivery details (if any) but many a times I have notice that when user try to deactivate an old item (not used for months /years) , they get same error message even when there are no open order lines , in such scenario my recommendations are to just check in the delivery details table (WSH_DELIVERY_DETAILS) and see if there are any record for this item that has Release Status as
1. Back Order
2. ready to released
If so, then that is the culprit, Cancel those delivery details and try to deactivate the item and now it should work.
If you are thinking why on earth order line is closed/cancelled but the delivery detail is still open/ready to release/back ordered ,then believe it is not worth to spend time on this as this is a data corruption (Unless you have 100+ records of this type).
So just cancel the delivery details /Lines and then deactivate the item
Hope that helps