Featured Image Not showing in Posts

I did a bit of looking around and found something in the theme. To solve your issue, go to to Appearance>Editor and on the right, click to content-single.php and look for this:

<!--
<div class="featured-image-single">
			<?php if (has_post_thumbnail() )
				the_post_thumbnail();
				?>
		</div>
-->

Now remove the <!-- at the beginning and the --> at the end. This will solve your problem. You no longer will need the plugin you had installed.

Hopefully that solves your problem