Wordpress：即使添加了支持，也无法设置特色图像元框

I've been adding featured images to my post for a while back but after the installation of a plugin (I think) the Set Featured Image meta box disappeared from Edit or Add New Post page in admin tab. I've re-installed wordpress but I can't get that featured image box back.

I have also added support for it in my theme's functions.php:

add_theme_support( 'post-thumbnails' );

Navigate to the top of the post page and click Screen Options. There should be a drop down with a lot of options like in the image attached. Make sure that Featured Image is selected.