themes-wordpress/perenne/patterns/404.php

<?php
/**
 * Title: A 404 page
 * Slug: perenne/404
 * Inserter: no
 */

declare( strict_types = 1 );
?>

<!-- wp:paragraph {"style":{"typography":{"lineHeight":"1.4"}},"fontSize":"small"} -->
<p class="has-small-font-size" style="line-height:1.4"><?php echo esc_html__( 'Oops! That page can’t be found.', 'perenne' ); ?></p>
<!-- /wp:paragraph -->