Hi Rajesh,
the error "OBJECT_UNKNOWN" usually means, that the IDoc type (or one of its segments) does not exist in the ABAP DDIC of the backend system, where PI is trying to lookup the IDoc definition.
However, please be aware: in older SAP releases IDOCTYPE_READ_COMPLETE used to have the "bug", that it also returned this error, if the SAP user calling it did not have the appropriate permissions for accessing the DDIC... See note 974537 (which also mentions the required authorization object for calling this function module).
Best Regards, Ulrich