themes-wordpress/beep/patterns/404.php

<?php
/**
 * Title: 404 page
 * Slug: beep/404
 * Inserter: no
 */
?>

<!-- wp:heading {"textAlign":"left","level":1,"anchor":"oops-that-page-can-t-be-found"} -->
<h1 class="wp-block-heading has-text-align-left" id="oops-that-page-can-t-be-found"><?php echo esc_html__( 'Error: 404 >> Not Found.', 'beep' ); ?></h1>
<!-- /wp:heading -->