Error During Booking

Hi,

I am getting the following error when i click on the order for processing:

Fatal error: Uncaught Error: Call to undefined function mb_strtolower() in /var/www/html/tw_wordpress/wp-content/plugins/bring-fraktguiden-for-woocommerce/pro/booking/views/class-bring-booking-order-view.php:122 Stack trace: #0 /var/www/html/tw_wordpress/wp-content/plugins/bring-fraktguiden-for-woocommerce/pro/booking/views/class-bring-booking-order-view.php(84): Bring_Booking_Order_View::render_start(Object(Bring_WC_Order_Adapter)) #1 /var/www/html/tw_wordpress/wp-admin/includes/template.php(1343): Bring_Booking_Order_View::render_booking_meta_box(Object(WP_Post), Array) #2 /var/www/html/tw_wordpress/wp-admin/edit-form-advanced.php(677): do_meta_boxes(Object(WP_Screen), ‘normal’, Object(WP_Post)) #3 /var/www/html/tw_wordpress/wp-admin/post.php(198): include(‘/var/www/html/t…’) #4 {main} thrown in /var/www/html/tw_wordpress/wp-content/plugins/bring-fraktguiden-for-woocommerce/pro/booking/views/class-bring-booking-order-view.php on line 122