Use “page” instead of “post”
'post_type' => 'page', // Set this to post or page
$args = array(
'id' => 'featured-image-2',
'post_type' => 'page', // Set this to post or page
'labels' => array(
'name' => 'Featured image 2',
'set' => 'Set featured image 2',
'remove' => 'Remove featured image 2',
'use' => 'Use as featured image 2',
)
);
