Display a div only in some author’s single posts

is_author() isn’t working as you expect because it checks whether the current query is for an existing author archive page.

If you’re placing your code in single.php inside the loop, you can use this instead:

<?php if( $post->post_author == '15' || $post->post_author == '16' ) { ?>
    <div>Your code here</div>
<?php } ?>

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