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CiviCRM Starterkit is installed as a Drupal installation profile, so it's got the civicrm module directory inside /profiles/civicrm_starterkit/modules rather than /sites/all/modules.  CiviCRM is assuming that you can go up two levels from the folder where it's currently running and then over to the default folder to find civicrm.settings.php.  That's not the case here.

A couple of things to do, then:


To deal with this in the short run, you might copy your settings file into code/profiles/default as a short-term hack.
File a bug report because obviously something's making an inaccurate assumption.
Consider whether using Pantheon and civicrm-starterkit is giving you any benefit.  Just because something can work doesn't mean it's worth the extra effort dealing with a quirky host and an unmaintained distribution.