Mpdf\MpdfException: Unable to create output file /pdf_using_mpdf/

The only controller method associated with a route this module implements doesn't take any action.

  /**
   * Generate a PDF file from an entity.
   */
  public function generate($node) {
    // TODO: remove anything dependent on node, everything must be
    // specific to Entity level as a whole in general.
    return [];
  }

Are you sure you are using this module, and not another one? See what the project page says.

Because the maintainer of pdf_using_mpdf was unresponsive and could not add me as a co-maintainer, this is the Drupal namespace I chose to get the D8 version. However, module namespace is still pdf_using_mpdf.

**Update**
I'm now added as the maintainer of original project and soon move the code base to that repository.