The email composition screen that appears is controlled by iOS and is not controlled by OnSong in any form. Because of this, there is likely an issue in your email account settings that is preventing iOS from sending an email. This could happen if you've deleted the default Mail app, or if you do not have an email account configured.
We would recommend contacting Apple Support if you need assistance in getting the default Mail app to send outgoing email. https://support.apple.com/contact