Codeigniter 3 MY_Controller not found

The error is as follows, by extending the Register class of the MY_Controller to create the user login, it throws me the following error. Any ideas?

He says he has not found the class, but even adding it does not find it.

Driver for registration in .php

<?php
defined('BASEPATH') OR exit('No direct script access allowed');

class Register extends MY_Controller
{

    public function index()
    {
        $this->load->library('form_validation');
        $this->form_validation->set_rules('first_name', 'First name','trim|required');
        $this->form_validation->set_rules('last_name', 'Last name','trim|required');
        $this->form_validation->set_rules('username','Username','trim|required|is_unique[users.username]');
        $this->form_validation->set_rules('email','Email','trim|valid_email|required');
        $this->form_validation->set_rules('password','Password','trim|min_length[8]|max_length[20]|required');
        $this->form_validation->set_rules('confirm_password','Confirm password','trim|matches[password]|required');

        //En caso de que no pueda registrarse correctamente lo redirecciona al index de nuevo
        if($this->form_validation->run()===FALSE)
        {
            $this->load->helper('form');
            $this->render('register/index_view');
        }
        else
        {
            $first_name = $this->input->post('first_name');
            $last_name = $this->input->post('last_name');
            $username = $this->input->post('username');
            $email = $this->input->post('email');
            $password = $this->input->post('password');

            $additional_data = array(
                'first_name' => $first_name,
                'last_name' => $last_name
            );

            $this->load->library('ion_auth');
            if($this->ion_auth->register($username,$password,$email,$additional_data))
            {
                $_SESSION['auth_message'] = 'The account has been created. You may now login.';
                $this->session->mark_as_flash('auth_message');
                redirect('user/login');
            }
            else
            {
                $_SESSION['auth_message'] = $this->ion_auth->errors();
                $this->session->mark_as_flash('auth_message');
                redirect('register');
            }
        }
    }
}