The below code, inserted in your wp-config.php file, will log all errors, notices, and warnings to a file called debug.log in the wp-content directory. It will also hide the errors so they do not interrupt page generation.
this code you must have to insert BEFORE /* That's all, stop editing! Happy blogging. */ in the wp-config.php file.
// Enable WP_DEBUG mode
define('WP_DEBUG', true);
// Enable Debug logging to the /wp-content/debug.log file
define('WP_DEBUG_LOG', true);
// Disable display of errors and warnings
define('WP_DEBUG_DISPLAY', false);
@ini_set('display_errors',0);
// Use dev versions of core JS and CSS files (only needed if you are modifying these core files)
define('SCRIPT_DEBUG', true);
sorce: https://codex.wordpress.org/Debugging_in_WordPress
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