File Upload error

[eluser]hrg.vincent[/eluser]
Controller

Code:

$this->showHead();

$config['upload_path'] = './upload/';  
$config['allowed_types'] = 'xlsx';

$this->load->library('upload', $config);
$this->upload->initialize($config);
    
if (!$this->upload->do_upload()){
$data = array('msg' => $this->upload->display_errors());  
}
else {
$data['upload_data'] = $this->upload->data('flex_file');
}

$this->load->view('flex/thanks');

any wrong from my code, as I can't upload the file to the folder upload.
it keep go to the part in below (error):

Code:

if (!$this->upload->do_upload()){
$data = array('msg' => $this->upload->display_errors());  
}