Why is Subinventory field disabled in manual planned order line?

Summary:

Use case:

We want to be ablet o create manual planned order for a BUIY item in planner workbench and assign a subinventory to that line. We want the subinventory to be populated for the Requisition that is released from workbench.

We are unable to assign a subinventory as the filed is greyed out (disabled). What other option is available to release manual order from workbench with subinventory assigned for the Requisition to pick up.

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