WHMCS integration fatel error Version

Hey I am getting an error with the WHMCS integration plugin the error is below.

Warning: chmod(): Operation not permitted in /var/www/vhosts/clarcloud.com/httpdocs/wp-content/plugins/whmcs-wordpress-integration/whmcs-integration.php on line 351 Fatal error: Call to undefined function mcrypt_encrypt() in /var/www/vhosts/clarcloud.com/httpdocs/wp-content/plugins/whmcs-wordpress-integration/whmcs-integration.php on line 441

And it is located at clarcloud.com/system

Thanks for your help