Thanks Indrajit and Rajesh for your suggestions.
1) I did the mapping of all the steps again and re-deployed the BPM. but same error.
2) Rajesh, InspireStdRequest (in screenshot of context object) is the first local variable/container to receive the message from Pi and it is also missing the value of "attribute" though I see it mapped one to one in mapping of trigger event.
Next, i am thinking of redoing the whole BPM from scratch -- from creating a new workspace hoping that this might resolve the (unknown) issue.
Any advice?
Thanks,
Latika.