You attempted to browse the contents of a virtual HTTP repository. However, the intended way of use is the configuration of APT and related package management systems to retrieve the software packages through this service.

To configure APT for use of Apt-Cacher NG you need to... 
EITHER: Configure APT to use a HTTP proxy by specifying it in apt.conf or related configuration files, see apt.conf manual page for details. Server and Port need to match the values used to visit this page. For example, edit /etc/apt/apt.conf (or create a new file called like /etc/apt/apt.conf.d/00aptproxy) and add the line:
Acquire::http::Proxy "http://192.168.1.171:3142";