The upload path does not appear to be valid in codeigniter when upload image

I'm trying to make crud with upload image. When I try to upload it keeps returning failed. I put var_dump it shows false when I put code to show error it returns:

"The upload path does not appear to be valid"

I've been searching for the solution and I found nothing. Can some one help me to find the error:

This is my controller :

 public function saveReimburse()
{
    validate_submitted_data(array(
        'nama' => 'required',
        'category_reimburse_id' => 'required',
        'amount' => 'required|numeric',
        'date_reimburse' => 'required',
        // 'photo' => 'required'
    ));
    // data
    $data = [
        'nama' => $this->input->post('nama'),
        'category_reimburse_id' => $this->input->post('category_reimburse_id'),
        'amount' => $this->input->post('amount'),
        'date_reimburse' => $this->input->post('date_reimburse'),
        'photo' => $_FILES['photo'],
        // 'status'=> $this->input->post("PENDING"),
        // 'nama' => $this->input->post('nama'),
    ];


    // condition
    $date = date('Y-m-d');
    $date = strtotime($date);
    $date = strtotime('-7 day', $date);

    if ($data['date_reimburse'] < date('Y-m-d', $date)) {
        echo json_encode(array('succes' => FALSE, 'message' => 'Max Reimburse was 1 week ago'));
    } else {
        // var_dump($data);
        // exit;
        if ($data['photo'] = "") {
        } else {
            $config = [
                'upload_path' => is_dir('assets/reimburse/') . 'assets/reimburse/',
                'allowed_types' => 'jpg|png|gif',
                'overwrite' => TRUE
            ];
            $this->load->library('upload', $config);
            $upload = $this->upload->do_upload('photo');

            var_dump($config);
            print_r($this->upload->display_errors());exit;

            var_dump($upload);exit;
            if (!$upload) {
                json_encode(array('success' => FALSE, 'message' => 'Failed Upload'));
                redirect('Reimburse/index', 'refresh');
            } else {
                $this->upload->data('file_name');
                $save = $this->reimburseModel->saveReimburse('reimburse', $data);
                var_dump($data);exit;
                if (!$save) {
                    echo json_encode(array('success' => FALSE, 'message' => 'Failed to reccord'));
                } else {
                    redirect('Reimburse/index', 'refresh');
                    echo json_encode(array('success' => TRUE, 'message' => 'Reimburse Success'));
                }
            }
        }
    }
}

You have to use base_path for physical path in codeigniter.

$base_path = $this->config->item('base_path');

Then Use $base_path as your project folder root directory and you can access any folder from your project folder to upload images.

Like. $base_path.'/'.'upload/';