Issue:
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
/etc/apt/sources.list
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
Reference:
https://linoxide.com/debian/configure-sources-list-debian-9/