Fatal error

After I installing the plugin and activate it and I go and choose the user I want to restrict. There on that page is this error:

“Fatal error: Call to undefined function mb_convert_encoding() in /home/cnoctavi/public_html/wp-content/plugins/restrict-author-posting/restrict-author-posting.php on line 145”

Instead of the possibility to choose the restricted category…

The page I need help with: [log in to see the link]