thanks for your response guys. The issue of error in plan HTTP adapter was that that the adpater between PI & ECC system was set as HTTP adapter and not XI adapter.
Now we have another issue, when the data comes in from the third party through SOAP sender channel, the channel shows up as successful but the message doesn't reach Integration server. In the Message Monitoring, for component adapter engine, the messages are showing as successful for request (third party to ERP) and response (ERP to third party) but there is no message in integration server. What could be the reason that mesages can't be seen in integration server, neither there is any evidence of message getting into ERP system. The SOAP sender synchronous channel is set with Adapter Engine as Central Adapter Engine.