Bootstrapping fails with Drupal requirement error [1h]

[AronNovak]

 [ExecStack] Executing terminus remote:drush project.dev -- si server --no-interaction --existing-config
 [ExecStack] Running terminus remote:drush project.dev -- si server --no-interaction --existing-config
 [warning] This environment is in read-only Git mode. If you want to make changes to the codebase of this site (e.g. updating modules or plugins), you will need to toggle into read/write SFTP mode first.
Warning: Permanently added '[appserver.dev.abc-d372-48fb-aec4-def.drush.in]:2222,[34.173.145.32]:2222' (RSA) to the list of known hosts.
 You are about to:
 * DROP all tables in your 'pantheon' database.

 [notice] Starting Drupal installation. This takes a while.
 [notice] Performed install task: install_select_language
 [notice] Performed install task: install_select_profile
 [notice] Performed install task: install_load_profile

In install.core.inc line 2329:
                                                                               
  Settings file:The <em class="placeholder">Settings file</em> is not writabl  
  e.<br /><br />The Drupal installer requires write permissions to <em class=  
  "placeholder">./sites/default/settings.php</em> during the installation pro  
  cess. The <a href="https://www.drupal.org/server-permissions">webhosting is  
  sues</a> documentation section offers help on this and other topics. 

Which is unexpected, especially that the installation through the web (install.php) goes through.

[AronNovak]

  Default settings file:The default settings file does not exist.<br /><br />  
  The Drupal installer requires that the <em class="placeholder">./sites/defa  
  ult/default.settings.php</em> file must not be deleted or modified from the  
   original download.