Could you please disable all plugins that are not related to our theme and provide us your admin and FTP access so we can check it? Be sure that you have added that PHP line to the functions.php file in your child theme and it is activated.
It seems to be something wrong with your Facebook APP. Be sure that you have followed our documentation steps https://xtemos.com/docs/woodmart/faq-guides/configure-facebook-login/
Try to recreate the APP and check again. If it will not work, add our account to your Facebook APP administrators so we can check