The mcrypt library has been declared "depreciated" since PHP 7.1 (replaced with OpenSSL). You can either downgrade PHP 7.2 or 7.3 via cPanel > MultiPHP or consult with script vendor.
PHP Version with mcrypt support:
PHP 7.1
PHP 7.0
PHP 5.6
Mcrypt extension missing in PHP 7.2 and PHP 7.3
