Can’t activate Fatal error

Fatal error: Uncaught Error: Call to undefined function mb_internal_encoding() in /home/prommsch/public_html/wp-content/plugins/inactive-user-deleter/inactive-user-deleter.php:27 Stack trace: #0 /home/prommsch/public_html/wp-content/plugins/inactive-user-deleter/inactive-user-deleter.php(936): InactiveUserDeleter->__construct() #1 /home/prommsch/public_html/wp-admin/includes/plugin.php(2141): include(‘/home/prommsch/…’) #2 /home/prommsch/public_html/wp-admin/plugins.php(175): plugin_sandbox_scrape(‘inactive-user-d…’) #3 {main} thrown in /home/prommsch/public_html/wp-content/plugins/inactive-user-deleter/inactive-user-deleter.php on line 27

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