This happens on devices when there's no appropriate mail app installed or it's not configured correctly.
Billdu uses the native iOS Mail app to send emails. Please make sure your native iOS Mail app is installed on your device and configured correctly. You will find more information about how to set up the mail app here.
When you email an invoice, Billdu looks for all available mail apps in your system and displays it to you. If nothing is displayed then there is either no appropriate mail app or your Android system didn't provide the information. You may want to install a new mail app or reinstall the old one, the information about your mail apps will be updated in your device.