Custom taxonomy.php not working

I noticed that siteurl.com/series does not work I get my 404 page

This is the correct behaviour. There is no content to display at this URL in WordPress. You need to provide a specific term so that WordPress can retrieve posts with that term.

Notice that in a fresh install of WordPress siteurl.com/category/ returns a 404, even if you have categories.

When I try siteurl.com/series/test-series and it works it is only pulling one post but in the series even though it should be displaying all of them.

The problem is likely that you have not built taxonomy-series.php correctly.

The first problem is that even though you’ve performed a query with WP_Query, you aren’t actually looping over the the results to output each one. Your code is written to only output once. Regardless, you shouldn’t be using WP_Query anyway.

When viewing a series at siteurl.com/series/test-series, WordPress will automatically query the correct posts for you. To display them you shouldn’t be making your own query. You should use The Loop.

So taxonomy-series.php should look like this:

<?php get_header(); ?>
    <div class="container">
        <div class="row">  
            <h2><?php single_cat_title(); ?></h2>    

            <?php while ( have_posts() ) : the_post(); ?>
                <div class="col-sm-6 card-block-img">
                    <?php $image = wp_get_attachment_image_src( get_post_thumbnail_id(), 'single-post-thumbnail' ); ?>
                    <a href="https://wordpress.stackexchange.com/questions/328534/<?php the_permalink(); ?>"><img src="<?php echo $image[0]; ?>"></a>
                </div>

                <div class="col-sm-6">
                    <h3><a href="https://wordpress.stackexchange.com/questions/328534/<?php the_permalink(); ?>"><?php the_title(); ?></a></h3>
                </div>
            <?php endwhile; ?>
        </div><!-- END ROW -->
    </div><!-- END CONTAINER -->
<?php get_footer(); ?>