Why .mo .po files are not working with my site?

As per Theme Handbook ⇒ Internationalization, you should call load_child_theme_textdomain() from within the after_setup_theme hook:

function load_greattheme_textdomain() {
    load_child_theme_textdomain( 'greattheme', get_stylesheet_directory() . '/languages' );
}
add_action( 'after_setup_theme', 'load_greattheme_textdomain' );

Make sure you name your *.mo and *.po files properly (follow the link above).