Hello Stella,
Thanks for writing in!
You are seeing a PHP warning message. it may have came from the theme or one of the plugins. Usually a warning message appear when a variable is null or not yet initialize. You can get rid of this message by disable the debud mode. You can do this by opening wp-config.php and adding
define('WP_DEBUG', false);
/* That's all, stop editing! Happy blogging. */
When you revisit the 404 page, you should no longer be seeing any warning message. Meanwhile, I noticed that you are using a custom 404 extension. Please make sure that you are up to date. If nothing is helping, provide us access to your site so that we can check your settings. Please create a secure note with the following info:
– Link to your site
– WordPress Admin username / password
To know how to create a secure note, please check this out: https://theme.co/docs/getting-support

Regards.