Incomplete requierements ?

After install, emptying cache and export in XLSX:

The website encountered an unexpected error. Please try again later.
Error: Class 'PhpOffice\PhpSpreadsheet\Spreadsheet' not found in Drupal\webform_xlsx_export\Plugin\WebformExporter\XlsxExporter->createExport() (line 42 of modules/contrib/webform_xlsx_export/src/Plugin/WebformExporter/XlsxExporter.php).
Drupal\webform_xlsx_export\Plugin\WebformExporter\XlsxExporter->createExport() (Line: 694)
Drupal\webform\WebformSubmissionExporter->writeHeader() (Line: 680)
Drupal\webform\WebformSubmissionExporter->generate() (Line: 130)
Drupal\webform\Controller\WebformResultsExportController->index(Object)
call_user_func_array(Array, Array) (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 582)
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 124)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext(Array, Array) (Line: 97)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 151)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 68)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 57)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1) (Line: 106)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1) (Line: 85)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 52)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 693)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)

No documentation, no requirements... what do i Miss?

I put phpspreadsheet library in libraries folder with no more luck...
Any help to do this module working would be greatly appreciated!