To install PacketFence under Debian 9.0 (Stretch), you simply have to add the Inverse repository under /etc/apt/sources.list.d/ by adding the file packetfence.list
echo 'deb http://inverse.ca/downloads/PacketFence/debian stretch stretch' \
wget -O - https://inverse.ca/downloads/GPG_PUBLIC_KEY | sudo apt-key add -
sudo apt-get update
sudo apt-get install packetfence
Once apt-get finished to install all the packages, reboot your server and after please fire up your Web browser and go to http://<ip_packetfence>:1443/configurator to complete your PacketFence configuration.