NONCE_SALT is a WordPress constant that is used to generate secure tokens known as “nonces.” Nonces are used to guard against specific security risks such as Cross-Site Request Forgery (CSRF) attacks. These are added to WordPress URLs and forms to guarantee that user activities are authentic and not fraudulent.
When the constant is not specified or set in the WordPress wp-salt.php file, the error message “undefined constant NONCE_SALT” appears. This file includes crucial WordPress installation settings and options, such as security keys and salts.
The NONCE_SALT Error

How to fix this?
To resolve this error, you should ensure that the NONCE_SALT
constant is properly defined in your wp-salt.php file. The wp-salt.php file should contain a section that looks like this:
define('NONCE_SALT', 'some-random-string-here');
The value 'some-random-string-here'
should be replaced with a unique and secure random string of characters. You can generate a secure string ( Salts ) using an online random string generator.
It will randomly generate the salts, like this:

The above example is not to be used. Generate your own unique salts here.
Once generated, paste and replace the whole content in your wp-salt.php file. Your issue and error should now disappear once the file is saved.
If you encounter this error despite having the NONCE_SALT
defined in the wp-salt.php
file, double-check the syntax of the definition and make sure there are no typos or syntax errors in the file.
Remember that if you’re not familiar with making changes to your WordPress installation, it’s always a good idea to back up your files before making any modifications. Additionally, seek assistance from someone experienced with WordPress.