Please update the while loop with this:
It will print the thumbnail url for you
** POST FETCHING ARGUMENTS **
<?php
/**** Slider Call Function ****/
function callTheSlider()
{
$args = array('post_type'=> 'expro_slider', 'post_status' => 'publish', 'order' => 'DESC');
?>
<ul>
<?php
wp_reset_query();
$query = new WP_Query($args);
while($query->have_posts()) : $query->the_post();
if(has_post_thumbnail()) { ?>
<li>
<?php the_post_thumbnail(); ?>
</li>
<?php }
elseif($thumbnail = get_post_meta($post->ID, 'image', true)) { echo 12323; ?>
<li>
<img src="https://wordpress.stackexchange.com/questions/264101/<?php echo $thumbnail; ?>" alt="<?php the_title(); ?>" title="<?php the_title(); ?>" />
</li>
<?php } endwhile;
?>
</ul>
<?php
}
?>