The Quantity field in Physical Inventory journal line is non-editable. While this makes sense for lines that are suggested by the calculate inventory, it does not allow to add lines which were not suggested.
Example: Item A has a calculated quantity of 1 for Bin B01. During the count, we discover that the item is not in B01 so we set that to zero. However we find item A in Bin B10. To handle this we need to add line to the journal. If we do this, we are not allowed to edit the Qty. (Physical) so we need to edit the quantity.