Php versions – Class 'MpdfMpdf' not found

I am using mpdf 8.1 with yii2 and php version is 5.6.40 for pdf report download. It’s works fine in local environment. But class mpdf is not found in live server with same php version.

mpdf library is uploaded in vendor directory and path is vendor/mpdf/mpdf. And I’m using like these,

use MpdfMpdf;
class ReportController extends Controller
{
    public function actionPdfUsageReport()
    {
          $content = "<div>Hello</div>";
          $marginValue = PdfSettings::GetTabularReportMarginSetting();
          $pdf = new Mpdf($marginValue);
          $stylesheet = file_get_contents(Yii::getAlias('@webroot') . "/css/mpdfstyletables.css");
          $pdf->WriteHTML($stylesheet, 1);
          $pdf->SetProtection(array('print'));
          $pdf->SetTitle("Title");
          $pdf->SetAuthor("Author.");
          $pdf->SetDisplayMode('fullpage');
          $pdf->WriteHTML($content);
          return $pdf->Output('Usage Summary.pdf', 'I');
    }
}

System information
Apache/2.4.6 (Red Hat Enterprise Linux) OpenSSL/1.0.2k-fips PHP/5.6.40