check in your theme functions.php file if you have this – add_theme_support( 'post-thumbnails' ); If not add this code there and see if it works then.
If you have to modify the functions.php file, it is best to create a child theme first. See how – https://codex.wordpress.org/Child_Themes
Try switching to another theme and see if you will see the Featured Image option. In case it is a theme related problem it would be best to contact the Lotus theme developers. Lotus theme is a premium theme and they have own forums where you could ask.