No matching template found when using single.php

I am building my own WordPress theme. I have the front page working. However, when I try to view a test post full of Lorem Ipsum, I get the "No matching template found" error.

I have the following code in single.php

<?php
    get_header();

    if( have_posts()){
        while( have_posts() ){
            the_post();
            the_content();
        }
    }

    get_footer();
?>

single.php is under directly my custom theme’s root folder.

From what I understand, the page should just display the content, albeit without much styling. I navigated to the post using the "view post" link on the post edit page if that matters.

Just to be sure, I also placed the same code in page.php, index.php, and page.php

No success. No matter what I try, I still get "No matching template found" when navigating to the test post.

What am I missing here? Is there a way to troubleshoot the template hierarchy?

Thanks in advance.