REQUIRED: Could not find the comment-reply script enqueued. See: Migrating Plugins and Themes to 2.7/Enhanced Comment Display

[sebastiaandegeus]

 <?php if ( is_singular() ) wp_enqueue_script( "comment-reply" ); ?>

[robertnicjoo]

add this into your functions.php

function newborn_enqueue_comments_reply() {
if( get_option( 'thread_comments' ) ) {
wp_enqueue_script( 'comment-reply' );
}
}
add_action( 'comment_form_before', 'newborn_enqueue_comments_reply' );