It appears you have an issue with your server and mysqli not being installed properly. You will have to open a support ticket with them to debug it.
I created a test page you can show them to help them see the issue. It has the code:
<?php
ini_set("display_errors",1);
echo(function_exists("mysqli_init")?"mysqli is working now":"mysqli is not installed");
$PowerCMSConnection = mysqli_init();
?>
If you show them that page, they should be able to fix it for you.