How to use Multisite WordPress functions in a different Single WordPress

I have two setup – Single WP & Multisite WP Network

I want to use the multisite’s wpmu_create_user function on the one custom file which is located on the root of single wp setup. I have tried below code but it throws Error establishing a database connection.

<?php
$wp_load_loc = “/opt/lampp/htdocs/mulsite/wp-load.php”;
require_once($wp_load_loc);
echo $wpdb->prefix;
$user = wpmu_create_user(‘test@test.com’, ‘123456’, ‘test@test.com’);
add_user_to_blog( 1, $newuser, ‘subscriber’ );
add_user_to_blog( 2, $newuser, ‘subscriber’ );
echo ‘Userid-‘ . $user;
?>

I have also tried including ms-settings.php and ms-load.php but doesn’t work.

After a lot of debugging, I have checked to include another single wp’s wp-load.php and it works (Below code)! So the issue seems to the multisite configuration which defined in the wp-config file.

<?php
ob_start();
$wp_load_loc = “/opt/lampp/htdocs/singlewp/wp-load.php”;
require_once($wp_load_loc);
echo $wpdb->prefix;
?>

I have spent many hours on such an issue. Kindly look into and please suggest the solution as soon as possible.

Thanks in advance!