Previous Post button not showing up

Your problem is with the URL to the button image, called “prev.png”. Most likely you need to add get_stylesheet_directory_uri() to your image URL in navigation.php. If the button is in a folder called images inside the theme folder, you would need:

<img src="<?php echo get_stylesheet_directory_uri(); ?>/images/prev.png">

Still a little confused. The code for my navigation site is

<?php
/**
 * You can change navigation in this is file
 *
 * @package WordPress
 * @subpackage constructor
 */
?>
<nav class="navigation">
    <?php if (is_singular()) : // Whether is single post, is a page, or is an attachment ?>
        <div class="alignleft"><?php next_post_link('%link', '<span>?</span> %title') ?></div>
        <div class="alignright"><?php previous_post_link('%link', '%title <span>?</span>') ?></div>
    <?php elseif (function_exists('wp_pagenavi')) : // Plugin pagenavi ?>
        <?php wp_pagenavi(); ?>
    <?php else: // Default page navigation ?>
        <div class="alignleft"><?php next_posts_link(__('<span>?</span> Older Entries', 'constructor')) ?></div>
        <div class="alignright"><?php previous_posts_link(__('Newer Entries <span>?</span>', 'constructor')) ?></div>
    <?php endif; ?>
</nav>

Where would I add the get_stylesheet_directory_uri

Which of the Constructor themes are you using? The problem may be with the template itself (some of them have the missing image file, some don’t)