You’re very likely running that outside of the loop.
To confirm, try the following:
$post_id = get_the_ID();
$current_rank = get_post_meta( $post_id, 'current_rank', true);
echo 'Post ID is: ' . $post_id;
echo '<br> Current Rank is: ' . $current_rank;
Are you getting post ID printed on the page? Is the current rank showing empty? Then the current_rank
meta value for this post is definitely empty.