Pass a comment id through url and append the comment post’s slug in the url

Unless you declare $wp_rewrite as a global in your test function, you’ll be trying to access the local variable $wp_rewrite. This, of course, won’t work.

Add the line global $wp_rewrite;:

<?php
add_action('init', 'test');
function test()
{
    global $wp_rewrite;
    $reply_struct="/reply/%reply_id%";
    $wp_rewrite->add_rewrite_tag('%reply_id%', '([^/]+)', 'reply_id=$matches[1]');
    $wp_rewrite->add_permastruct('reply_id', $reply_struct, false);
}