When you are first making a WordPress theme from scratch, Featured Images are not enabled by default. To enable Featured Images, simply add the following line to your functions.php file:
add_theme_support('post-thumbnails');
Now you will be able to use Featured Images on blog posts and pages.
Custom Post Types
To enable Featured Images in Custom Post Types, simply add ‘thumbnail’ to the “supports” array item:
'supports' => array('thumbnail')