The original design philosophy of warehouse receiving is that this is a function for the warehouse to perform receiving. The warehouse should not have any business in entering receiving unit cost. Therefore, during the Add mode of warehouse receiving, the user is not able to enter the unit cost. The system simply defaults the unit cost based on purchase order line item unit cost. Admin users may need to review the unit costs before they get posted. So admin users can use the Change mode to modify unit cost.
But for whatever reason, if you let your admin users (instead of the warehouse) enter warehouse receiving transactions, you may find the restriction cumbersome. Therefore, we have added a new flag in Global Setup -> Dist -> PO Warehouse Receiving:
9. Allow to Enter Unit Cost in Add Mode?
The default is "N." If you change it to "Y," then you will now able to enter unit cost in the PO Warehouse Receiving Add mode. See sample screen below: