ajax - Failed To start Session Using PHP and Codeigniter

I have problem with starting session Using codeigniter framework trying to make a login but I can't login.

this is my controller code

public function login()
{
    if($this->input->post("login"))
    {
        $this->load->library('form_validation');
        $this->form_validation->set_rules("username","UserName",'required|min_length[3]|max_length[30]|alpha');
        $this->form_validation->set_rules("password","password",'required|min_length[3]|max_length[30]');
        if ($this->form_validation->run())
        {
            $username=$this->input->post("username");
            $password=$this->input->post("password");
            $this->load->model("usersmodel");
            if($this->usersmodel->mainlogin($username,$password))
            {
                $session_data=array(
                    'username' => $username
                );
                $this->session->set_userdata($session_data);
                redirect(site_url("Users/dashboard"));


            }else{
                $this->session->set_flashdata('error', 'Error With loGIN ');
                redirect(site_url("Users/login"));
            }

        }else{
            //error for validation
        }
    }

    $data['pagetitle']="Log in";
    $this->load->view("users/login",$data);
}
public function dashboard()
    {
        if($this->session->userdata('username')==!'')
        {
            echo"welcom";

        }
        $this->load->model("usersmodel");
        $this->load->view("template/admin/dashboard");
    }
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this is my model

public function mainlogin($username,$password)
{
    /*
    $this->db->select('*');
    $this->db->from("users");
    $this->db->where('username',$username);
    $this->db->where('password',$password);
    $sql=$this->db->get();
    return $sql->row();
    */
    $this->db->where('username',$username);
    $this->db->where('password',$password);
    $sql=$this->db->get('users');
    if($sql->num_rows() >0)
    {
        return true;
    }else{
        return false;
    }
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this is my view

<div >
  <div id="container">

    <div >
      <div ><label ><?php echo validation_errors()?></label></div>
<form method="post" action="<?php echo site_url("Users/add")?>" enctype="multipart/form-data">
  <div >
    <label for="recipient-name" >Full Name :</label>
    <input type="text"  name=" fullname" id="fullname" placeholder="please insert fullname" autocomplete="off" required="required">

  </div>

  <div >
    <label for="recipient-name" >UserName :</label>
    <input type="text"  name="username" id="username" placeholder="please insert user name" autocomplete="off" required="required" >
  </div>
  <div >
    <label for="message-text" >Password:</label>
    <input type="password"  name="password" id="password" placeholder="please insert yout password" autocomplete="new-password" required="required" >
    <i aria-hidden="true"></i>
  </div>
  <div >
    <label for="recipient-name" >Email :</label>
    <input type="text"  name="email" id="email" placeholder="please insert email" autocomplete="off" required="required" >
  </div>
  <div >
    <label for="recipient-name" >User Type :</label>
    <select name="groubid" >
      <option value="1">Administartor</option>
      <option value="0">User</option>
      <option value="2">Maker</option>
      <option value="3">cheker</option>

    </select>
  </div>
  <div >
    <label for="exampleInputFile">Image :</label>
    <input type="file" name="image" id="image">

  </div>

  <button type="submit"  name="adduser" value="adduser" >add</button>
</form>
    </div>
  </div>
</div>
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