mix: not found command

[ThetNaungSoe]

###Error

2016-12-22T09:20:47.448746+00:00 heroku[web.1]: Starting process with command mix kitto.server
2016-12-22T09:20:49.263901+00:00 app[web.1]: bash: mix: command not found
2016-12-22T09:20:49.364231+00:00 heroku[web.1]: State changed from starting to crashed
2016-12-22T09:20:49.351672+00:00 heroku[web.1]: Process exited with status 127
2016-12-22T10:02:18.449094+00:00 heroku[web.1]: State changed from crashed to starting
2016-12-22T10:02:22.057330+00:00 heroku[web.1]: Starting process with command mix kitto.server
2016-12-22T10:02:23.744474+00:00 app[web.1]: bash: mix: command not found
2016-12-22T10:02:23.803986+00:00 heroku[web.1]: Process exited with status 127
2016-12-22T10:02:23.815612+00:00 heroku[web.1]: State changed from starting to crashed
2016-12-22T10:08:39.929366+00:00 heroku[router]: at=error code=H10 desc="App crashed" method=GET path="/" host=hidden-gorge-21362.herokuapp.com request_id=a5ff33f6-3202-4300-a4b8-1b08f505fe54 fwd="103.25.77.156" dyno= connect= service= status=503 bytes=
2016-12-22T10:08:40.810089+00:00 heroku[router]: at=error code=H10 desc="App crashed" method=GET path="/favicon.ico" host=hidden-gorge-21362.herokuapp.com request_id=e9821410-a203-4542-b1b1-6cc3d4fca943 fwd="103.25.77.156" dyno= connect= service= status=503 bytes=