Fatal error when importing

I copy and base the html code from salesforce and it gives me an import error:

Fatal error: Call to undefined function mb_detect_encoding() in salesforce-wordpress-to-lead/lib/simple_html_dom.php on line 1234

Is there something I am doing wrong or a way to fix it?

Thanks

https://www.ads-software.com/plugins/salesforce-wordpress-to-lead/