I have added the module and trying to add the Add On but I get the following error with Drupal 9.
EDIT: I also posted this to the Slack board and one person had this comment that might help.
"There is code in the regular add to cart formatter to detect layout builder, probably needs to be added to PADO as well. I'd be surprised if there wasn't an issue."
Warning: array_flip(): Can only flip STRING and INTEGER values! in Drupal\Core\Entity\EntityStorageBase->loadMultiple() (line 261 of core/lib/Drupal/Core/Entity/EntityStorageBase.php).
Drupal\Core\Entity\EntityStorageBase->loadMultiple(Array) (Line: 245)
Drupal\Core\Entity\EntityStorageBase->load(NULL) (Line: 34)
Drupal\commerce_pado\PadoLazyBuilders->addToCartWithAddOnsForm(NULL, '_custom', , 'field_add_sleeve_logo', )
call_user_func_array(Array, Array) (Line: 101)
Drupal\Core\Render\Renderer->doTrustedCallback(Array, Array, 'Render #lazy_builder callbacks must be methods of a class that implements \Drupal\Core\Security\TrustedCallbackInterface or be an anonymous function. The callback was %s. See https://www.drupal.org/node/2966725', 'exception', 'Drupal\Core\Render\Element\RenderCallbackInterface') (Line: 786)
Drupal\Core\Render\Renderer->doCallback('#lazy_builder', Array, Array) (Line: 356)
Drupal\Core\Render\Renderer->doRender(Array, 1) (Line: 201)
Drupal\Core\Render\Renderer->render(Array, 1) (Line: 157)
Drupal\Core\Render\Renderer->Drupal\Core\Render\{closure}() (Line: 578)
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 158)
Drupal\Core\Render\Renderer->renderPlain(Array) (Line: 172)
Drupal\Core\Render\Renderer->renderPlaceholder('callback=commerce_pado.lazy_builders%3AaddToCartWithAddOnsForm&args%5B0%5D&args%5B1%5D=_custom&args%5B2%5D=&args%5B3%5D=field_add_sleeve_logo&args%5B4%5D=&token=m7-Qlip-WFvYEZTjtkhnGog1IHVFUoDqC76xwrYZSa4', Array) (Line: 693)
Drupal\big_pipe\Render\BigPipe->renderPlaceholder('callback=commerce_pado.lazy_builders%3AaddToCartWithAddOnsForm&args%5B0%5D&args%5B1%5D=_custom&args%5B2%5D=&args%5B3%5D=field_add_sleeve_logo&args%5B4%5D=&token=m7-Qlip-WFvYEZTjtkhnGog1IHVFUoDqC76xwrYZSa4', Array) (Line: 547)
Drupal\big_pipe\Render\BigPipe->sendPlaceholders(Array, Array, Object) (Line: 305)
Drupal\big_pipe\Render\BigPipe->sendContent(Object) (Line: 112)
Drupal\big_pipe\Render\BigPipeResponse->sendContent() (Line: 381)
Symfony\Component\HttpFoundation\Response->send() (Line: 20)