It appears there is a syntax error with this line:
if ( have_posts() ) while ( have_posts() ) : the_post();
You are missing a colon after ( have_posts() ).
If it’s not a theme related issue as mentioned in the comments if you change it to:
“if ( have_posts() ) : while ( have_posts() ) : the_post();“
Also, the endwhile should come BEFORE the endif.
After making these changes it should work.
See Example Here:
<div class="row">
<div class="col-md-8">
<?php if ( have_posts() ) : while ( have_posts() ) : the_post(); ?>
<div class="blog-post">
<div class="single-post">
<div class="post-thumb">"><img src="<?php the_post_thumbnail_url(); ?>" alt="blog thumb" /></div>
<div class="blog-single-content">
<div class="blog-list-content">
<p class="post-meta">Posted By <a href="#"> <?php the_author(); ?> </a></p>
<h3 class="blog-title"> <?php the_title(); ?> </h3>
<?php the_content(); ?>
</div> <!-- class="blog-list-content" -->
</div> <!-- class="blog-single-content" -->
</div> <!-- class="single-post" -->
</div> <!-- class="blog-post" -->
<?php endwhile; ?>
<?php endif; ?>
<?php comments_template(); ?>
I always check my PHP with a code checker here: https://phpcodechecker.com/. I have found it helps a lot when debugging my code.
Hopefully, this works for you!