Hi Amarnath,
When you send production planning machine breakdown alert to APO system, you need to specify RFC destination of APO system in required field. It would be like APP or APPCLNT001 or APOCLNT000 (as per client system id), as Steffen said you can find through SM59 or in IMG -> Event Management -> General Settings in SAP Event Management -> Define RFC Connection to Application System or check table RFCDES-RFCDEST to find and enter in activity parameter APO Destination field.
Regards
GGOPII