I believe you need to add `__FILE__` to your [`plugins_url()`][1] function. It may not be calling the right location. It should look like this:

    add_action( 'wp_enqueue_scripts', 'wpse_238154_register_plugin_styles' );
    
    function wpse_238154_register_plugin_styles() {
        wp_enqueue_style( 'fh-plugin' , plugins_url( 'nf-plugin/includes/f-hide.css', __FILE__ ) );
    }


  [1]: https://codex.wordpress.org/Function_Reference/plugins_url