This lab involves a front-end and back-end server, and the front-end server doesn't support chunked encoding. The front-end server rejects requests that aren't using the GET or POST method.
To solve the lab, smuggle a request to the back-end server, so that the next request processed by the back-end server appears to use the method GPOST.
Using Burp Repeater, issue the following request twice:
POST / HTTP/1.1
The second response should say: Unrecognized method GPOST.