I need help resolving an error when entering my remote whmcs host in the wp whmcs integration plug-i

I get this error everytime I enter my remote host information which is http://xuckovpn.com/members. I've tried entering it in multiple formats and still get the error. the error message is" Fatal error: Call to undefined function mb_strrchr() in /home/xuckovpn/public_html/wp-content/plugins/whmcs-wordpresss-integration/lib/url_to_absolute.php on line 119"
if you can help me resolve this that would be great thank you.