Making Requestor field required conditionally when Invoice is not matched with PO

Summary:

Trying to make Requestor field mandatory based on the invoice if not matched with PO. Found exact match as , however I don't see the attribute PoNumber.attributeValue in the binding parameter drop down list, any additional details for clarification will be very helpful. I tried with other filed on the binding variable and it is working correctly, but we need to put the condition for PoNumber only

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