PHP error in testing

Receiving this error after successfully completing a test transaction:

Fatal error: Call to undefined function Stripe\Util\mb_detect_encoding() in /home/koolestate/public_html/wp-content/plugins/stripe/libraries/stripe-php/lib/Util/Util.php on line 111

Not sure if this has anything to do with the redirect settings in the plugin. Also not getting any notification of this test transaction in my Stripe dashboard. Can you help, please?

https://ww.wp.xz.cn/plugins/stripe/