Error while loading wordpress site "Fatal error: Uncaught Error: Call to undefined function twentytwentyone_the_html_classes() in /home

I am facing fatal error while loading wordpress site. Error is:

Fatal error: Uncaught Error: Call to undefined function twentytwentyone_the_html_classes() in

......(here in the middle shows some links).... and at the end it shows in bold

public_html/wp-content/themes/twentytwentyone/header.php on line 16

There has been a critical error on this website.

I am not experience in wordpress. I need to resolve this error. here is my site URL https://ijazmedia.com/ I have tried to add below code in header.php file but it did not work.

?php 
  if (!defined('ABSPATH')) exit;
  get_header(); 
?

I am using parent theme. Any help will be much appreciated.