Hi,
You will get this error if you set SERVER_MODE to "cgi" in server_conf.py but not actually run the experiment as CGI (which would involve visiting the URL of your experiment on a web server under, say, Apache) but instead run it in stand-alone mode using python server.py. If you run your experiment on your machine (and not on a web server) then you are running it in “toy” mode, so set SERVER_MODE to "toy"
Jeremy