Error Message:
There was a failure executing the send pipeline: "Microsoft.BizTalk.DefaultPipelines.XMLTransmit" Source: "XML assembler" Send Port: "Target\Path" Reason: The root element is missing.
Solution:
When ever this error comes its mean your input xml document is not schema validated.
Get Valid Input XML Document
Open your mapping file and right click on its and the click on Test Map. Get XML document from the Output Window and pass this XML document as a input source.