Post Slider Not Working

Hi,
After using post slider in page builder, The page turns white and the following errors are added to the log file !
Please check the problem to fix the problem
good luck

[30-Jul-2020 19:51:19 UTC] PHP Fatal error:  Uncaught Error: Call to undefined function mb_strimwidth() in /home/cpfir255/public_html/wp-content/themes/enfold/framework/php/function-set-avia-backend.php:768
Stack trace:
#0 /home/cpfir255/public_html/wp-content/themes/enfold/config-templatebuilder/avia-shortcodes/postslider/postslider.php(662): avia_backend_truncate('\n\xD8\xA8\xD9\x87 \xD9\x88\xD8\xB1\xD8\xAF\xD9\xBE\xD8...', 60, ' ', '\xE2\x80\xA6', true, '')
#1 /home/cpfir255/public_html/wp-content/themes/enfold/config-templatebuilder/avia-shortcodes/postslider/postslider.php(469): avia_post_slider->html()
#2 /home/cpfir255/public_html/wp-content/themes/enfold/config-templatebuilder/avia-template-builder/php/shortcode-template.class.php(1041): avia_sc_postslider->shortcode_handler(Array, '', 'av_postslider', Array)
#3 /home/cpfir255/public_html/wp-includes/shortcodes.php(343): aviaShortcodeTemplate->shortcode_handler_prepare(Array, '', 'av_postslider')
#4 [internal function]: do_shortcode_tag(Array)
#5 /home/cpfir255/public_html/wp-includes/shortcodes in /home/cpfir255/public_html/wp-content/themes/enfold/framework/php/function-set-avia-backend.php on line 768
[30-Jul-2020 19:53:55 UTC] PHP Fatal error:  Uncaught Error: Call to undefined function mb_strimwidth() in /home/cpfir255/public_html/wp-content/themes/enfold/framework/php/function-set-avia-backend.php:768
Stack trace:
#0 /home/cpfir255/public_html/wp-content/themes/enfold/config-templatebuilder/avia-shortcodes/postslider/postslider.php(662): avia_backend_truncate('\n\xD8\xA8\xD9\x87 \xD9\x88\xD8\xB1\xD8\xAF\xD9\xBE\xD8...', 60, ' ', '\xE2\x80\xA6', true, '')
#1 /home/cpfir255/public_html/wp-content/themes/enfold/config-templatebuilder/avia-shortcodes/postslider/postslider.php(469): avia_post_slider->html()
#2 /home/cpfir255/public_html/wp-content/themes/enfold/config-templatebuilder/avia-template-builder/php/shortcode-template.class.php(1041): avia_sc_postslider->shortcode_handler(Array, '', 'av_postslider', Array)
#3 /home/cpfir255/public_html/wp-includes/shortcodes.php(343): aviaShortcodeTemplate->shortcode_handler_prepare(Array, '', 'av_postslider')
#4 [internal function]: do_shortcode_tag(Array)
#5 /home/cpfir255/public_html/wp-includes/shortcodes in /home/cpfir255/public_html/wp-content/themes/enfold/framework/php/function-set-avia-backend.php on line 768