Not working – TwentyTen

This plugin seems to be just what I’ve been looking for. However, it’s not working on my pages with the TwentyTen theme. I’ve checked and header.php does have

“echo get_the_post_thumbnail( $post->ID, ‘post-thumbnail’ );”

I’ve tried it with [feslider] in the body of the post as well as without.

A page that I’ve used to become with Feslider is https://www.pngyoung.com/HikingTest/tapia-canyon-hike-04-06-2020/

Thank you for your assistance.
cheers,
george