Warning message about header (Woocommerce plugin?)

I am pretty sure the following error message is related to my Woocommerce plugin. When Woocommerce plugin is deactivated, error message does not appear. Please advise what I should do to remove the error message! Here is the code around line 63 of the php file

Warning: Cannot modify header information – headers already sent by (output started at /home/morima5/public_html/wp-content/themes/mystile/functions.php:11) in /home/morima5/public_html/wp-content/plugins/woocommerce/classes/class-wc-session-handler.php on line 63

$cookie_value = $this->_customer_id . ‘||’ . $this->_session_expiration . ‘||’ . $this->_session_expiring . ‘||’ . $cookie_hash;

setcookie( $this->_cookie, $cookie_value, $this->_session_expiration, COOKIEPATH, COOKIE_DOMAIN, false, true );

// Actions
add_action( ‘woocommerce_cleanup_sessions’, array( $this, ‘cleanup_sessions’ ), 10 );