Pretty expensive result for such a basic mistake by the Engineers. The question re the Pilot is if during his walk around preflight could or should he have seen the cover. Also begged is whether there were lengthy warning ribbons attached to the blank that would have facilitated Engineers and/or Pilot seeing it.
Airplanes can be replaced...even at extremely high cost....but human lives are irreplaceable.
Hopefully an frank evaluation of safety procedures looking for other waiting uh oh's will be done.