This error frequently occurs when your application’s secret key base is not set and you attempt to run it in production mode. If you’re are still just in the testing and development phase, the quickest way to resolve the issue is to set passenger_app_env in your Nginx configuration to development mode:

server {
    listen 80 default_server;
    server_name example.com;
    passenger_enabled on;
    passenger_app_env development;
    root /home/rails/app/public;
}

When running in production, you’ll want to make sure that the secret key base is correctly configured. First, make sure your config/secrets.yml file contains:

production:
  secret_key_base: <%= ENV["SECRET_KEY_BASE"] %>

Next, generate a secret key base by running the command rake secret in your Rails project. Now export that value as the SECRET_KEY_BASE environmental variable.

For more info, check out:

• How To Deploy Rails Apps Using Passenger With Nginx on CentOS 6.5