Error establishing a database connection

Hello, I am trying to get MU Domain Mapping work on my wordpress which I host on a server from Vultr.

I installed the plugin already and basically followed all instructions, like it shows in this video here: https://www.youtube.com/watch?v=0R7toDXu0W4

The problem occurs after adding the folder mu-plugins and pasting the domain_mapping.php

Once I add this, I get the following error:

Error establishing a database connection
If your site does not display, please contact the owner of this network. If you are the owner of this network please check that MySQL is running properly and all tables are error free.

Database tables are missing. This means that MySQL is not running, WordPress was not installed properly, or someone deleted wp_site. You really should look at your database now.

What do I do now? Read the bug report page. Some of the guidelines there may help you figure out what went wrong. If you’re still stuck with this message, then check that your database contains the following tables:

wp_users
wp_usermeta
wp_blogs
wp_signups
wp_site
wp_sitemeta
wp_registration_log
wp_blog_versions

Ofcourse I added the sunrise.php into the wp-content folder.

When editing the wp-config.php tho, It looks way different than on the video.

I pasted the define( ‘SUNRISE’, ‘on’ ); in several places, always above this comment /* That’s all, stop editing! Happy blogging. */, but nothing is working.

My wp-config looks like this. Maybe the issue is, because its a wordpress plugin installed on a server?

<?php

// ** MySQL settings ** //
/** The name of the database for WordPress */
define('DB_NAME', 'EDITED');

/** MySQL database username */
define('DB_USER', 'wpuser69773');

/** MySQL database password */
define('DB_PASSWORD', 'EDITED');

/** MySQL hostname */
define('DB_HOST', 'localhost');

/** Database Charset to use in creating database tables. */
define('DB_CHARSET', 'utf8');

/** The Database Collate type. Don't change this if in doubt. */
define('DB_COLLATE', '');

define('AUTH_KEY',         'EDITED');
define('SECURE_AUTH_KEY',  'EDITED');
define('LOGGED_IN_KEY',    'EDITED');
define('NONCE_KEY',        'EDITED');
define('AUTH_SALT',        'EDITED');
define('SECURE_AUTH_SALT', 'EDITED');
define('LOGGED_IN_SALT',   'EDITED');
define('NONCE_SALT',       'EDITED');

$table_prefix = 'wp_';

define( 'FORCE_SSL_ADMIN', false );

/* That's all, stop editing! Happy blogging. */

/** Absolute path to the WordPress directory. */
if ( !defined('ABSPATH') )
	define('ABSPATH', dirname(__FILE__) . '/');

/** Sets up WordPress vars and included files. */
require_once(ABSPATH . 'wp-settings.php');

I really need to make this work, but unfortunately I don’t have any Idea what I could do. I try since days, but it just doesn’t work.

Thanks a lot in advance!

https://www.ads-software.com/plugins/wordpress-mu-domain-mapping/

EDIT: I am using wordpress 4.4.2, but the plugin says following.

Required: 3.1 or higher
compatible till: 4.0.10

Probably this is the issue? Can I just downgrade to use it?