Why system still need Requested or Promised Ship Date even I already provide Requested Delivery Date

Summary: Requested Delivery Date doesn't work after enabling BMT.

Content (required): We create PO and select planned item and already assign the Requested Delivery Date on the PO line, but system still pops up this message "You must enter either a requested date or a promised date for planned items", which means it still need Requested Ship Date or Promised Ship Date. Why?

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