WordPress PHP syntax doesn’t seem to be working correctly

get_the_post_thumbnail() returns the full html output of the image, including the image <img /> tags. You want just the image src. Try this:

<div class="leftBox">
    <?php
    $query = new WP_Query( 'pagename=jour' );
    // The Loop
    if ( $query && $query->have_posts() ) :
        while ( $query->have_posts() ) : $query->the_post();
            if ( has_post_thumbnail() ) {
                $img_src = wp_get_attachment_image_url( get_post_thumbnail_id( get_the_ID() ), 'thumbnail' );
                 ?>
                <div class="entry-content" style="background: url('<?php echo esc_url( $img_src ); ?>') no-repeat;">
                    <a href="https://wordpress.stackexchange.com/questions/223955/<?php echo get_permalink(); ?>" title="Go to <?php echo the_title(); ?>"><p><?php the_title(); ?></p></a>
                </div>
                <?php
            }
        endwhile;
    endif;
    /* Restore original Post Data */
    wp_reset_postdata();
    ?>
</div>