WP_DEBUG is a PHP constant (a permanent global variable) that can be used to trigger the "debug" mode throughout WordPress. It is assumed to be false by default in the wp-config.php file of WordPress.
To turn on debug mode:
- Option 1: download your wp-config.php via FTP, open with a text editor and change false to true for the WP_DEBUG option as shown here
- Option 2: Login to your Hosting account as explained in Hosting Account Access and navigate to the "Files" tab. Click on the httpdocs folder or the domain name in question within File Manager as shown here.
Navigate to the root of your Wordpress installation and click the small arrow that appears when hovering over the wp-config.php file and choose "Edit in Text Editor" as shown here. Then change false to true as shown here. Make sure to click/select the "Save" button at the bottom of the editing form.
** Remember to turn off debug mode by setting this to false after your troubleshooting as non-fatal errors may display on the screen/site.
You can find the full documentation on Wordpress debug mode at https://codex.wordpress.org/Debugging_in_WordPress