Test SSO without an CAS or OAUTH2 Server - C0D3D3V/Moodle-DL GitHub Wiki
You have to change this file: /admin/tool/mobile/launch.php
Just comment out Line 63 like this:
if (empty($SESSION->justloggedin) and
$typeoflogin != tool_mobile\api::LOGIN_VIA_BROWSER and
$typeoflogin != tool_mobile\api::LOGIN_VIA_EMBEDDED_BROWSER) {
// throw new moodle_exception('pluginnotenabledorconfigured', 'tool_mobile');
}