$this->form_validation->error_array() not work

please help, i'll try this code in my localhost is working but not in may hosting server.

PHP Code:

public function register() {
         $this->load->library('form_validation');
           $this->output->set_content_type('application_json');
           $this->form_validation-set_rules('name','UserName','required|min_length[4]|max_length[16]|is_unique[user.username]');
           if ($this->form_validation->run() == false){
               $this->output->set_output(json_encode([$this->form_validation->error_array()]));
               return false;
           }
  } 


Please turn on your error output and post that with this
try ini_set('display_errors', 1); in your index.php file.

Without seeing the error, could be
is_unique[user.username] is not working do to not being able to connect to database properly.
or
$this->output->set_output(json_encode([$this->form_validation->error_array()])); is not parsing correctly.