Raj,
By default WAS 6.1 enables global security and since you are trying to stop the server without passing the actual credentials your are getting this error.

2 ways to resolve this issue.
1. try to stop your server by killing the Java process.
2. Try to stop using the command stopServer.sh/bat serverName -username sss -password pwd ( substitute the username and password with your windows/unix login credentials )

Thanks
K Balu