Looks like you’re trying to display comments for a single post that has comments disabled.
You should find if ( comments_open() ) statement just one line below.
Paste your code there and it will work just fine:
<?php if ( comments_open() ) : ?>
<div class="comments-link">
<?php comments_popup_link( '<span class="leave-reply">' . __( 'Leave a reply', 'twentytwelve' ) . '</span>', __( '1 Reply', 'twentytwelve' ), __( '% Replies', 'twentytwelve' ) ); ?>
</div><!-- .comments-link -->
<?php comments_template( '', true); ?>
<?php endif; // comments_open() ?>
Also, I can’t find PHP starting & ending tag in your example, but I suppose it’s just a typo? 🙂
Cheers,
I hope it helps.