Ok,
Sorry for the missed ].
I have made an orchestration with your schema in an orchestration.
To verify that the Xpath is correct, check the following properties in the node of the xsd schema : Instance Xpath.
If it is correct, verify that the node exist. The Xpath command can modify a node-value but can’t create a node.
In your construct message, do you use a Transform with a map or do you try to set the message directly? (If you use a transform map,be sure that you create the node).
If necessary, post more details on your orchestration