Fresh installation on the same server where another core version was installed gives error
After you install Drupal, automatically Drupal set sites/default to 555 and settings/default/settings.php to 444 , So it means they haven't deleted . 

my suggestion is to change the permission for them, them delete settings.php (and make sure you delete it correctly ), and when you got insure of deleting everything Setup Drupal 7.56 again.