$seo_audit = get_stylesheet_directory() . '/seo-audit.php';
if (file_exists($seo_audit)) {
    require_once $seo_audit;
}
<?xml version="1.0" encoding="UTF-8"?>
<?xml-stylesheet type="text/xsl" href="https://onlineplapuestas.com/wp-sitemap-index.xsl" ?>
<sitemapindex xmlns="http://www.sitemaps.org/schemas/sitemap/0.9"><sitemap><loc>https://onlineplapuestas.com/wp-sitemap-posts-page-1.xml</loc></sitemap></sitemapindex>