Solution:
In order to enable debug mode in wordpress you can use the global variable ‘WP_DEBUG’:
define( ‘WP_DEBUG’, true );
define( ‘WP_DEBUG’, false );
WP_DEBUG_LOG causes all errors to also be saved to a debug.log log file inside the /wp-content/ directory.
define( ‘WP_DEBUG_LOG’, true );
Here’s the link to the WordPress documentation on debugging your site.
Regarding the error-msg it looks like an attribute used in ../wp-content/themes/kallyas-child/js/zn_script_child.js in line 3 is undefined onLoad (when the window is loaded). I hope that will help you to find the problem/issue.