<?php
get_header();
?>
<div id="primary" class="content-area">
<main id="main" class="site-main">
<?php
// Start the Loop.
while ( have_posts() ) :
the_post();
get_template_part( 'template-parts/content/content', 'page' );
// If comments are open or we have at least one comment, load up the comment template.
if ( comments_open() || get_comments_number() ) {
comments_template();
}
endwhile; // End the loop.
?>
</main><!-- #main -->
</div><!-- #primary -->
<?php
get_footer();
このようなコードになっている思いますので、echoを追加します。
<?php
get_header();
?>
<div id="primary" class="content-area">
<main id="main" class="site-main">
<?php
// ↓↓↓↓追加↓↓↓↓
echo "これはテストです。";
// ↑↑↑↑追加↑↑↑↑
// Start the Loop.
while ( have_posts() ) :
the_post();
get_template_part( 'template-parts/content/content', 'page' );
// If comments are open or we have at least one comment, load up the comment template.
if ( comments_open() || get_comments_number() ) {
comments_template();
}
endwhile; // End the loop.
?>
</main><!-- #main -->
</div><!-- #primary -->
<?php
get_footer();