Error In Plugin Something mdf call internal encoding error

I recently migrated my website to new domain and i received the error as follow :

Fatal error: Uncaught Error: Call to undefined function Mpdf\mb_internal_encoding() in /home/skillsin/rkpsimmigration.com/wp-content/plugins/send-pdf-for-contact-form-7/mpdf
/vendor/mpdf/mpdf/src/Mpdf.php:1126 Stack trace: #0 /home/skillsin/rkpsimmigration.com/wp-content/plugins/send-pdf-for-contact-form-7/views/send-pdf-admin.php(329):
Mpdf\Mpdf->__construct() #1 /home/skillsin/rkpsimmigration.com/wp-content/plugins/send-pdf-for-contact-form-7/classes/send-pdf.php(198): include(‘/home/skillsin/…’) #2
/home/skillsin/rkpsimmigration.com/wp-includes/class-wp-hook.php(308): cf7_sendpdf->wpcf7pdf_dashboard_html_page() #3 /home/skillsin/rkpsimmigration.com/wp-includes/
class-wp-hook.php(332): WP_Hook->apply_filters() #4 /home/skillsin/rkpsimmigration.com/wp-includes/plugin.php(517): WP_Hook->do_action() #5 /home/skillsin/rkpsimmigration.com/
wp-admin/admin.php(259): do_action() #6 {main} thrown in /home/skillsin/rkpsimmigration.com/wp-content/plugins/send-pdf-for-contact-form-7/mpdf/vendor/mpdf/mpdf/src/Mpdf.php
on line 112

Please help me to solve this ?