what am i doing wrong

I made a category page – where i would like to have thumbnail images to my posts. But no images is showing.

What am i doing wrong

———-

<?php get_header(); ?>

<?php while (have_posts()) : the_post(); ?>

		<div class="col-md-3 post-item" <?php if ( in_category(16, $post) ) { echo "style='border: 1px solid #67BB22;'"; } ?>>
			<h2><a>" rel="bookmark" title="<?php printf(__('Permanent Link to %s'), the_title_attribute('echo=0')); ?>"><?php the_title(); ?><?php the_post_thumbnail('') ?></a></h2>
			<small>Skrevet d. <?php the_time(__('d/m/Y')) ?> <!-- by <?php the_author() ?> --> - <?php comments_popup_link(__('Ingen kommentarer ?'), __('1 Kommentar ?'), __('% Kommentarer ?'), '', __('Lukket for kommentarer') ); ?></small>

			<div class="post_thumbnail">
<?php get_the_post_thumbnail() ?>
</div>

			<div class="entry">
				<?php the_excerpt(__('L?s mere ?')); ?>
			</div>

			<p class="postmetadata"><?php edit_post_link(__('Rediger'), ''); ?>  <?php comments_popup_link(__('Ingen kommentarer ?'), __('1 Kommentar ?'), __('% Kommentarer ?'), '', __('Lukket for kommentarer') ); ?></p>
		</div>

	<?php endwhile; ?>

———–

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Hope you can help

Cheers Kimo