You must run 'session_start();' on each page where you use sessions. 'session_start();' must be the first line of code in your php page -
if (session_status() == PHP_SESSION_NONE) {
session_start();
}
require_once 'init.php';
include 'includes/head.php';
include 'includes/navigation.php';
use PHPMailer\PHPMailer\PHPMailer;
use PHPMailer\PHPMailer\Exception;