the_content() in single-{post-type}.php problem

First of all, you shouldn’t use get_posts on your CPT archive – WP already creates a query and selects posts that should be display in there. So your archive-pjesma.php should look like this:

<?php get_header(); ?>

<?php while ( have_posts() ) : the_post(); ?>
    <h2><a href="https://wordpress.stackexchange.com/questions/326484/<?php the_permalink(); ?>"><?php the_title(); ?></a></h2>
<?php // the_content(); ?>
<?php endwhile; ?>

<?php get_footer();

And you also should use simplified version of loop in your single-pjesma.php:

<?php get_header(); ?>

<?php if ( have_posts() ) : the_post(); ?>
<h1><?php the_title(); ?></h1>
<?php the_content(); ?>
<?php endif; ?>

<?php get_footer();