In case you didn't put your source code in the root folder and you didn't configure it in Artifakt console, you'll see this line in logs:
Parent directory /srv/www/drupal/releases/xxx/sites/default does not exist
Check if your Drupal source code is in the root of the Git Repository. If not, you'll have to define the sub-folder in the environment configuration like that:
After this change, try to launch your code build again.