My apologies for the late response. To print the Featured image on the single post page, you may want to add the code below to /loop-templates/content-single.php file.
<?php echo get_the_post_thumbnail( $post->ID, ‘large’ ); ?>
You can paste it to the 16th line of the file (just below the <header class=”entry-header”>), to print the thumbnail above post title.
Please contact me if there are any problems.
Thanks for your patience,