Getting 503 service unavailable when using session (on cloudways server)

Question

I'm using cloudways.com to develop my website (codeigniter) and I would like to use session to store some data.

I test my code on localhost, it's work well but when I upload it to server (which is on cloudways.com), it return 503 service unavailable. Please help me to solve this problem.

Here is my code. (session.php)

session_start();
$_SESSION['facebook_access_token'] = (string) 'flkdjsifojsodifjejfnlaskfe=';

and on home.php I use

session_start();
echo $_SESSION['facebook_access_token'];

on home.php getting 503 error.