Hi Siva,
Could you please check in debugging by keeping a break point on Fun.Module(B46B_DPO_TRANSFER( In side this function module there is one more Fm:BBP_PO_INBOUND(keep break point on this FM) )) and check the sy-subrc value ( if it is 1 = Communication failure, 2 = system failure).
IF Sy-subrc returns 1 means communication error, so check the RFC connections, If it returns 2 means system failure .. In that case you have to check the backend logs( based on time)..
All the best.
Regards,
Sankarbabu