Featured Image not showing in admin

You have misspelled the action hook name.

This:

add_action( 'after-setup_theme', 'ja_theme_setup' );

Should be:

add_action( 'after_setup_theme', 'ja_theme_setup' );

The full code:

add_action( 'after_setup_theme', 'ja_theme_setup' );
function ja_theme_setup() {
    add_theme_support( 'post-thumbnails');
}