Search with no expected result

from the errorlog

mod_fcgid: stderr: PHP Fatal error: Call to undefined function wpcx_get_wp_generated_thumb() in /home/xxxxxxxxxxxxxxxx/domains/yyyyyyyyyyyyyyyyyy/public_html/wp-content/themes/zzzzzzzzzzzzzzzzzzzzz/404.php on line 18

and line 18 in 404.php looks line this :

$thumb = wpcx_get_wp_generated_thumb("reference");

What to do ?

What theme is this?

It is “homemade” custom basic theme instructed by :

https://line25.com/tutorials/how-to-build-your-own-wordpress-theme