This is because no WP or Plugin files are defined if this file is loaded outside of WP. There isn’t any way that you can do what you are attempting to do without also loading WP. The best way to do this is to call and action function using admin-ajax.php. You can load this file without using AJAX and create an action that outputs what you want where you need it output.
https://codex.wordpress.org/AJAX_in_Plugins
