Getting node in theme_preprocess_node
As the exception message says, there is no english translation for that node. 

The node is already in the translation that Drupal determined is the best available translation for this page.

If you do want to get a specific translation, you need to check with hasTranslation() first or use \Drupal::service('entity.repository')-&gt;getTranslationFromContext($entity, $langcode);