Warning: Cannot modify header information – headers already sent by (output star

Warning: Cannot modify header information – headers already sent by (output started at /home/stuartrental/public_html/www.stuartrental.com/wp-content/themes/venedor/inc/content_types/post.php:711) in /home/stuartrental/public_html/www.stuartrental.com/wp-content/plugins/wpclef/includes/lib/wp-session/class-wp-session.php on line 124

can login to the site. but the site show the error, does not load.