"Older Entries" & "Newer Entries" doesn't appear. Code ignored?

What template file are you adding this to?

Hi esmi, maybe I don’t understand your question. The outputsite is https://www.conceptualisms.org/main/
And the whole code of this page is

<?php get_header(); ?>
<div id="inhalt" class="text_single">

<?php $custom_query = new WP_Query('category_name=books,texts,news'); // exclude category 9 bzw. reblog
while($custom_query->have_posts()) : $custom_query->the_post(); ?>

	<div <?php post_class(); ?> id="post-<?php the_ID(); ?>">
<h1 class="text_autor"><a href="<?php the_permalink(); ?>"><?php echo get_post_meta($post->ID, 'Autor', true); ?>,
			 <?php the_title() ?></a></h1></p>
			<p class="text_datum"><?php the_date();?></p>

		<?the_excerpt(); ?>
	</div>
<p>−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−</p>
<?php endwhile; ?>

<div class="text_datum">
<?php next_posts_link('&laquo; Older Entries') ?>
<?php previous_posts_link('Newer Entries &raquo;') ?>
</div>

</div>
<?php get_sidebar(); ?>
<?php get_footer(); ?>

Is that what you asked for?

Why are you using WP_Query()? Is this a custom page template?

Yes it is a costum page template, but it was not my decision. Unfortunately, the former programmer can’t work on it anymore. If there’s a better solution than WP_Query() that makes it easier to get the navigation, I’m open to it.

WP_Query() should only be used for secondary queries – not the main query. Try using pre_get_posts() instead.

using next-posts-link with custom query:

https://codex.www.ads-software.com/Function_Reference/next_posts_link#Usage_when_querying_the_loop_with_WP_Query

Thank you. I tried it out. I use the code, I just found here https://codex.www.ads-software.com/Pagination
and changed it a bit, so it should appear in my area “inhalt”, which means “content”:

<?php get_header(); ?>
<div id="inhalt" class="text_single">

<?php if ( have_posts() ) : ?>

<!-- Add the pagination functions here. -->

<!-- Start of the main loop. -->
<?php while ( have_posts() ) : the_post();  ?>

<!-- the rest of your theme's main loop -->

<?php endwhile; ?>
<!-- End of the main loop -->

<!-- Add the pagination functions here. -->

<div class="nav-previous alignleft"><?php next_posts_link( 'Older posts' ); ?></div>
<div class="nav-next alignright"><?php previous_posts_link( 'Newer posts' ); ?></div>

<?php else : ?>
<p><?php _e('Sorry, no posts matched your criteria.'); ?></p>
<?php endif; ?>

</div>

<?php get_sidebar(); ?>
<?php get_footer(); ?>

Now nothing appears at all. What did I do wrong?

Ok, I see that the code is just a basic structure.
So I found this https://codex.www.ads-software.com/the_loop and tried it out:

<?php get_header(); ?>

<div id="inhalt" class="text_single">
 <?php query_posts($query_string . '&cat=-40'); ?>
 <?php if ( have_posts() ) : while ( have_posts() ) : the_post(); ?>

 <div class="post">

 <!-- Display the Title as a link to the Post's permalink. -->
 <h2><a href="<?php the_permalink() ?>" rel="bookmark" title="Permanent Link to <?php the_title_attribute(); ?>"><?php the_title(); ?></a></h2>

 <!-- Display the date (November 16th, 2009 format) and a link to other posts by this posts author. -->
 <small><?php the_time('F jS, Y') ?> by <?php the_author_posts_link() ?></small>

  <div class="entry">
    <?php the_content(); ?>
  </div>

  <p class="postmetadata">Posted in <?php the_category(', '); ?></p>
 </div> <!-- closes the first div box -->

 <?php endwhile; else: ?>
 <p>Sorry, no posts matched your criteria.</p>
 <?php endif; ?>

</div>

<?php get_sidebar(); ?>
<?php get_footer(); ?>

But the only thing I get is one single post, but I already have 8. Why is the loop not working? Any suggestions? It looks so simple.
Is there no working simple loop with pagination out there for an archive of posts?
&cat=-40') is just to make this to have no effect, as there is no cat with the id 40.