MPDF在codeigniter（PHP 7.2版）中不工作

我在我的服务器上安装了mpdf，但是当我运行代码时，它会显示错误

An uncaught Exception was encountered
Type: Error

Message: Class 'Mpdf\Mpdf' not found

Filename: 
/home/jewelmarts/public_html/royal.chains/application/controllers/admin_panel/Collection.php

Line Number: 374

Backtrace:

File: /home/jewelmarts/public_html/royal.chains/index.php
Line: 317
Function: require_once

这是我的密码

 public function download_pdf($id){
    require(MPDF);
    $mpdf = new \Mpdf\Mpdf(['tempDir' => MPDF_TEMP_PATH]);
    $html = '<span>test</span>';
    print_r($html);die;
    $mpdf->AddPage();
    $mpdf->WriteHTML($html);
    $mpdf->Output($file,'I');
}

另外，请检查我安装mpdf的命令

请帮我解决这个问题

先谢谢你

define('MPDF',$_SERVER['DOCUMENT_ROOT'] . '/vendor/autoload.php');
define('MPDF_TEMP_PATH',$_SERVER['DOCUMENT_ROOT'] . '/vendor/mpdf/mpdf/tmp');