Bug report

Hi oanhtran1804,
This is not a bug nor is it an error – it is just a notice of an undefined variable.

This is nothing to worry about. If you need it so it does not display, you should disable notices (and make sure you do not have WP_DEBUG set to true on a live site).

The easiest way to turn off warnings and notices, is to set them to log to a file, but only for actual errors (so the file does not get overly large in size).

Here is code you can add to your wp-config.php file to create an error log in the root of your site:

Add just before the line that says if ( !defined('ABSPATH') ) :

ini_set('display_errors', 0);
ini_set('log_errors', 1);
ini_set('error_log', dirname(__FILE__) . '/ujrdfg8lakj-error-log.txt');
error_reporting( E_ALL  & ~E_NOTICE & ~E_WARNING );

Notice I called the error log file ujrdfg8lakj-error-log.txt? This makes it harder to guess the error log – otherwise anyone could easily view it. Call it whatever you want to call it.

Best regards,
Don

No problem.
Usually you can find and edit the wp-config.php file from your hosting control panel using a file manager program (most have them). It is located in the root of the site (sometimes in then public_html folder depending on your setup).

This link may help:
https://codex.ww.wp.xz.cn/Editing_wp-config.php

Be sure that you exercise caution when editing the file, as mistakes can cause your site to go down until you fix them.

If you just open it and add the code, you should be fine.

Side Note: you CAN add the code to your functions.php file as well, so that is also an option. Usually you can edit that under APPEARANCE/EDITOR and then selecting the functions.php file. Again, use caution when editing. Also recommend backing up the site as a precaution.

Best of luck,
Don