Unable to store data into the session in 3.1.8

$this->session->set_userdata('email'); won't do much
You *could* do $this->session->set_userdata($array);, which will save the email address as the value for the key 'email'.
Or you could $this->session->set_userdata('email', $array['email');

You are not using sessions correctly.
https://www.codeigniter.com/user_guide/l...ssion-data