Add featured image in single post

@sejar,

Try add this code to functions.php (use custom child theme or plugin FunctionsPHP).

function add_post_thumbnail() {
    $thumbnail = get_the_post_thumbnail( get_the_ID(), 'large', '' );
    if ( is_singular() ) : ?>
        <a href="<?php the_permalink() ?>" class="singular-thumbnail">
            <?php echo $thumbnail; ?>
        </a>
    <?php endif;
}
add_action( 'basic_before_content', 'add_post_thumbnail' );

https://wp-puzzle.com/docs/basic/show-thumbnail-on-single-posts.html