Hey Ankit,
very detailed you explain us which SAP function modules you are using. It would have been a little bit better if you had shown us something more of your coding surrounding your message statement.
Message ennn is causing an error in the environment of the SAP coding which embeds your coding during runtime. So pls. use the message statement with wnnn or snnn before you use the fm isu_msg_put; then you will see your 'error' message text. The SAP fm does not matter for that trick.
Best regards
Karl-Christian