Hello,
Maybe you are not connecting to the right URL. (The URL is the Moodle site where your courses are.) If you don't know it, please contact your teacher/trainer and they'll provide the right one.
If you are already trying the right URL, please contact your site administrator and refer to https://docs.moodle.org/36/en/Moodle_app_FAQ for more information.
Kind regards,
Isabel