source: twentynineteen/2.2/404.php

<?php
/**
 * The template for displaying 404 pages (not found)
 *
 * @link https://codex.wordpress.org/Creating_an_Error_404_Page
 *
 * @package WordPress
 * @subpackage Twenty_Nineteen
 * @since Twenty Nineteen 1.0
 */

get_header();
?>

        <div id="primary" class="content-area">
                <main id="main" class="site-main">

                        <div class="error-404 not-found">
                                <header class="page-header">
                                        <h1 class="page-title"><?php _e( 'Oops! That page can&rsquo;t be found.', 'twentynineteen' ); ?></h1>
                                </header><!-- .page-header -->

                                <div class="page-content">
                                        <p><?php _e( 'It looks like nothing was found at this location. Maybe try a search?', 'twentynineteen' ); ?></p>
                                        <?php get_search_form(); ?>
                                </div><!-- .page-content -->
                        </div><!-- .error-404 -->

                </main><!-- #main -->
        </div><!-- #primary -->

<?php
get_footer();