How to use add_settings_error in register_setting callback

Take a look at the add_settings_error prototype.

add_settings_error( $setting, $code, $message, $type );

The first argument is your settings name/key — or if your setting is on another page (eg general) it should be the page key. The second is whatever you’d like to add to the ID attribute, then error/updated message, and finally type. It doesn’t work because you’re using it incorrectly.

So you probably want…

<?php
add_settings_error(
    'your_setting_key', // whatever you registered in `register_setting
    'a_code_here', // doesn't really mater
    __('This is the message itself', 'wpse'),
    'error', // error or notice works to make things pretty
);

You also need to tell WordPress to display your settings errors. If it’s on a custom page, you’ll need to include settings_errors in the callback.

settings_errors('your_setting_key');