Per the FAQ, "The "Ended" and "Cancelled" TV Show Status now automatically disabled the "Add New Season" button. New seasons can't be created when a TV entry is set to Ended or Cancelled. For ongoing series, fixing the TV show status will also bring back the ability to create a new season. For ended/cancelled series with a missing season, you are allowed to temporarily change the TV Show Status back to "Returning Series". Just don't forget to set it back to Ended/Cancelled when you are done adding the missing content."

I set the status field back to Returning Series, so you should be able to add the missing season(s). Please don't forget to change it back to Ended when you're done adding seasons.