Hello Aniket,
sorry for the delay in answering.
Please go to Site Administration > Mobile app > Mobile authentication. In there you should change the Type of login and set it to "Via a browser window" or "Via an embedded browser". That way the app will open your Moodle site in the device browser or an embedded browser, authenticate the user in there, and then go back to the app to complete the authentication.
Kind regards,
Dani