Jai,
The step wm.ip.cm.handlers:processUnhandledDocument handles for any general exception caused in the Buyer-Seller converstation steps.
Please make sure in the “Starting Document” property of your conversation script is set as "PIP3A4 V1.1 Purchase order Request Document"to receive this document in the IS Administration Console.
So when TN receives the conversation using processDocument step conversationID will get populated and this will kicksoff the converstation.
HTH,