PHP errors

Im getting this error message:

Uncaught Error: Call to undefined function Mpdf\mb_internal_encoding() in /var/www/html/wordpress/wp-content/plugins/dk-pdf/vendor/mpdf/mpdf/src/Mpdf.php:1096 Stack trace: #0 /var/www/html/wordpress/wp-content/plugins/dk-pdf/includes/dkpdf-functions.php(164): Mpdf\Mpdf->__construct(Array) #1 /var/www/html/wordpress/wp-includes/class-wp-hook.php(286): dkpdf_output_pdf(Object(WP)) #2 /var/www/html/wordpress/wp-includes/class-wp-hook.php(310): WP_Hook->apply_filters(”, Array) #3 /var/www/html/wordpress/wp-includes/plugin.php(531): WP_Hook->do_action(Array) #4 /var/www/html/wordpress/wp-includes/class-wp.php(750): do_action_ref_array(‘wp’, Array) #5 /var/www/html/wordpress/wp-includes/functions.php(1105): WP->main(”) #6 /var/www/html/wordpress/wp-blog-header.php(16): wp() #7 /var/www/html/wordpress/index.php(17): require(‘/var/www/html/w…’) #8 {main} thrown in /var/www/html/wordpress/wp-content/plugins/dk-pdf/vendor/mpdf/mpdf/src/Mpdf.php on line 1096

Any ideas what might be going wrong? Clearly an environment issue where mDPDf isnt working somehow… ??

Many thanks anyone!