Let me try to answer your question - the “content.php” file should be available in “template-parts” directory.
Once confirmed, please locate and open “functions.php” file available in main theme directory. If file hasn’t been modified, you should have following line under line 43:
add_theme_support( 'post-thumbnails' );
If it’s not there, check the entire file as this line is essential for featured image to show up later. If there’s no such line at all, add it at the end of your “functions.php” file.
Now save the file and head over to “content.php”. Again, if file hasn’t been modified, look for line number 15. You should see following code there:
if ( is_singular() ) :
the_title( '<h1 class="entry-title">', '</h1>' );
the_title( '<h2 class="entry-title"><a href="' . esc_url( get_permalink() ) . '" rel="bookmark">', '</a></h2>' );
Now add following code just above the previous one:
if ( has_post_thumbnail() ) :
the_post_thumbnail( 'full' );
If you’ve followed all the steps correctly, featured image will be visible from now on just above post title. You will most likely have to adjust its appearance in “style.css” file later on.