Installing child theme

Hi, I have a theme installed on my wordpress site and all of the content is live. I want to use a child theme provided by the author to edit some of the code in the main theme. MY problem is when I try to install and upload the theme.

I get this error message?

“Unpacking the package…

Fatal error: Call to undefined function mbstring_binary_safe_encoding() in /var/www/vhosts/rescue2.co.uk/httpdocs/wp-admin/includes/class-wp-filesystem-ftpext.php on line 121″

It seems a very unique error message and I dont really know where to start

Any help much apperciated thanks