View file StuffPost/stuffpost/404.php

<?php get_header(); ?>
<div class="py-5 my-5 text-center container">
  <div class="row py-5 mb-5 justify-content-center">
    <div class="col-lg-6">
      <h1 class="display-4 c-1 font-weight-bold"><?php esc_html_e('Error 404!','stuffpost'); ?></h1>
      <p><?php esc_html_e("We couldn't find the page you are looking for.","stuffpost") ?></p>
      <hr class="my-5">
      <p class="small"><?php esc_html_e("Try to search something:","stuffpost") ?></p>
      <?php get_search_form() ?>
    </div>
  </div>
</div>
<?php get_footer(); ?>