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.