Purchase Order Field Mandate in Oracle fusion Sales Order screen

Summary:

Hi team,
We want to mandate ;Purchase Order field in Sales order field page.
When I logged in to sandbox the required field is showing disable?
when we select expression builder #bindings.CustomerPONumber.hints.Mandatory and click on Ok

getting this error : javax.faces.validator.ValidatorException: Returned value '' is unexpected for attribute 'Required'.

How can we mandate purchase order field from Sandbox?
Thank You

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