Interesting thread at the R-Pi Community.
 
First, User is installing Raspbian Stretch by SSH and can't. It seems that SSH is disabled on Stretch (and the last Jessie but I did not have this problem when I built the Astro Pi). To get ssh back turned on, after you installed Raspbian on your SD Card, you add a blank text file to the directory that Windows see called "ssh" or "ssh.txt" This turns on ssh on boot.
 
Second, Wifi and other options need to be configured on Stretch as well if you want it as a headless system.
 
These two things are odd, especially if you intend to run a headless system like with Raspbian Lite version (no GUI, only Text Console) - which is going to require ssh on it. But these are people who are running a full windows gui in their R-Pies. So why not add a keyboard and monitor to their R-Pies and configure it from there? That is what I did to get my Astro Pi running. Even the Lite Version lets you log in with a keyboard and monitor and run it like a secure DOS system.
 
Third - despite all this, there are several requests for logging in as the root user. Root User was disabled like on many systems (including Apple's OSX) and you have to log in through a normal user and use "sudo". But somebody put in the Linux codes to activate the Root User Account. The Root user Account was always there but it has been dormant. Activating it and giving it a password wakes it up. Problem if somebody hacks into your root account, that is it for your system.
 
All three subjects are on a single thread of the Newbie Section of the Raspberry Pi forum:
https://www.raspberrypi.org/forums/viewtopic.php?f=91&p=1227387&sid=cfb8 064a6a2cab7ba79384de830a1ba4