I would like to change the error message: "Invalid login details" into "This username or email already exists." I am using a plugin called Login Customizer and even if I deactivate it, the same error appears. Reference Picture
To change the error message, you can try modifying the code in the plugin that generates the error message. You can also try modifying the WordPress core files that handle user authentication, but this is not recommended as it may cause issues with your site and can be overwritten during updates.
You can try modifying the plugin's code by following these steps:
Navigate to the WordPress plugin directory on your server, which is typically located at /wp-content/plugins/.
Locate the Login Customizer plugin folder and open it.
Find the file that contains the code for the error message, which is typically called functions.php or login-customizer.php.
Open the file in a text editor and search for the error message text.
Replace the error message text with the new message you want to use.
Save the file and upload it back to the server, overwriting the existing file.
Test the login form to see if the error message has been updated.
Keep in mind that modifying the plugin's code may cause issues with your site if the plugin is updated or if there are conflicts with other plugins or themes. It is always recommended to backup your site before making any changes to the code.