Scheduled Task Error

Hi,

I am setting up a scheduled task and am getting this error:

PHP Notice:  Undefined index: SERVER_NAME in /var/www/html/admin/config/config.php on line 2

This is my config.php file (some details removed):

<?php
    switch($_SERVER['SERVER_NAME']) {

        case 'clientsite.loc':
            include(__DIR__.'/config.clientsite-loc.php');
            break;

        case 'clientsite.staging.co.uk':
            include(__DIR__.'/config.clientsite-staging.php');
            break;

        case 'dev.clientsite.co.uk':
            include(__DIR__.'/config.clientsite-dev.php');
            break;

        default:
            include(__DIR__.'/config.production.php');
            break;
    }

    define('PERCH_LICENSE_KEY', 'XXXXXX-XXXXXX-XXXXXX-XXXXXX-XXXXXX');
    define('PERCH_EMAIL_FROM', 'info@client-site.co.uk');
    define('PERCH_EMAIL_FROM_NAME', 'Client');

    define('PERCH_LOGINPATH', '/admin');
    define('PERCH_PATH', str_replace(DIRECTORY_SEPARATOR.'config', '', __DIR__));
    define('PERCH_CORE', PERCH_PATH.DIRECTORY_SEPARATOR.'core');

    define('PERCH_RESFILEPATH', PERCH_PATH . DIRECTORY_SEPARATOR . 'resources');
    define('PERCH_RESPATH', PERCH_LOGINPATH . '/resources');

    define('PERCH_HTML5', true);
    define('PERCH_TZ', 'UTC');

    define('PERCH_CUSTOM_EDITOR_CONFIGS', true);

The individual config files contain db details, PERCH_SCHEDULE_SECRET and PERCH_SITEPATH in each.

Any thoughts what could be causing the error?

This is my diagnostics:

    Perch Runway: 3.1.1, PHP: 7.0.30-0ubuntu0.16.04.1, MySQL: mysqlnd 5.0.12-dev - 20150407 - $Id: b5c5906d452ec590732a93b051f3827e02749b83 $, with PDO
    Server OS: Linux, apache2handler
    Installed apps: content (3.1.1), assets (3.1.1), categories (3.1.1), perch_forms (1.11), perch_twitter (4.0)
    App runtimes: <?php $apps_list = [ 'perch_twitter', 'perch_forms', 'casino36_crm' ];
    PERCH_LOGINPATH: /admin
    PERCH_PATH: /var/www/html/admin
    PERCH_CORE: /var/www/html/admin/core
    PERCH_RESFILEPATH: /var/www/html/admin/resources
    Image manipulation: GD
    PHP limits: Max upload 2M, Max POST 8M, Memory: 128M, Total max file upload: 2M
    F1: 3b606135b33e6a102526838f4152a807
    Resource folder writeable: Yes
    HTTP_HOST: dev.casino36.co.uk
    DOCUMENT_ROOT: /var/www/html
    REQUEST_URI: /admin/core/settings/diagnostics/
    SCRIPT_NAME: /admin/core/settings/diagnostics/index.php