Unable to change item UOM while creating internal PR through RSSP

Summary:

Unable to change item UOM while creating IMT PR.

Content (please ensure you mask any confidential information):

There more than 1000 items that are transferred with different UOMs in different transactions (Each/Packet/Carton). Primary UOM of an item is Each. User is trying to create an internal request (IMT PR) to Main warehouse to transfer 10 Cartons of this item. But item's primary UOM 'Each' is defaulted and cannot be edited while creating IMT PR (source as Inventory).

For the same item, if the source is 'Supplier', then UOM field is editable and the user is able to choose Carton as UOM. We need the same capability for IMT PR as well.