Once you change the wp-admin path and switch on the WP Ghost > Change Paths > Admin Security Hide the New Admin Path option, you can’t access the new admin URL as a visitor (if you are not logged in to your website).
You need to go to the new login path. After you log in, you will be redirected to your WordPress dashboard on the new admin path.
Read More: Change and Hide wp-admin Path
To change the 404 error message when accessing the New Admin Path, switch off the Hide the New Admin Path option.
Also, you can change the error with Front Page redirect from WP Ghost > Tweaks > Redirects.
Read More: Redirect Hidden Paths