You’ve installed Debian but during the installation you didn’t specify a good DNS server, or Debian didn’t pick up any of the sources you specified.
How when you run apt-get update nothing happens or you get an error because the sources are not configured properly.
The solution is to go into
And edit that file, adding the correct information. Typically you want to add something like:
deb http://deb.debian.org/debian stretch main
deb-src http://deb.debian.org/debian stretch main