Call Featured Image from Parent Page

I believe you want something like the following:

<?php 
if( has_post_thumbnail( $post->post_parent ) ) { 
  echo get_the_post_thumbnail( $post->post_parent );
}
?>

Calling the_post_thumbnail(); should return the current post thumbnail, whereas using get_the_post_thumbnail(); will allow you to specify the ID, using the parent ID in your case.

Function Reference for get_the_post_thumbnail()

Also, if you’re doing this outside of the loop, you’ll need the $post global variable, as seen here.