Please follow these instructions to install Mailtrack Add-on for Android:
1) Open the Mailtrack for Android page on your Android phone or in the Chrome browser on your computer.
2) Press or click the install button.
3) Select the email account you want to use to send tracked emails using your Gmail app.
4) Press the allow button to accept the permissions.
5) If the install has been successful you should receive a confirmation message saying that Mailtrack for Android Gmail has been installed.
6) Re-start the Gmail app on your phone. In order to do so, please, close it completely and open it again. If you don't know how close completely your Gmail app, please, switch off your phone and switch it on again.