Http 500 error after upgrading from Drupal 6.38 to Drupal 7.54

I upgraded from Drupal 6.38 to Drupal 7.54 following all instructions under https://www.drupal.org/docs/7/upgrade-to-drupal-7/upgrade-process. At completion (Step 13 run the update script) I was not able to access my website to make the update, getting "HTTP Error 500". I changed the settings.php as directed ($update_free_access = FALSE; to $update_free_access = TRUE;) still get the error when attempting to access my site to run update.php. Using FTP. Any assistance would be appreciated. Thanks