Can't Install Packages with apt-get | no installation candidate

Question

I'm using Raspbian Stretch with Desktop(v4.14.62-v7) and I can't seem to be able to install packages on my rPi.

For instance, when I type sudo apt-get install autoconf, I get:

Reading package lists... Done
Building dependency tree       
Reading state information... Done
Package autoconf is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Package 'autoconf' has no installation candidate

I've updated and upgraded but nothing seems to change.

Update

The output of apt policy:

Package files:
100 /var/lib/dpkg/status
 release a=now
500 http://archive.raspberrypi.org/debian stretch/ui armhf Packages
 release o=Raspberry Pi Foundation,a=stable,n=stretch,l=Raspberry Pi Foundation,c=ui,b=armhf
 origin archive.raspberrypi.org
500 http://archive.raspberrypi.org/debian stretch/main armhf Packages
 release o=Raspberry Pi Foundation,a=stable,n=stretch,l=Raspberry Pi Foundation,c=main,b=armhf
 origin archive.raspberrypi.org
500 http://raspbian.raspberrypi.org/raspbian stretch/rpi armhf Packages
 release o=Raspbian,a=stable,n=stretch,l=Raspbian,c=rpi,b=armhf
 origin raspbian.raspberrypi.org
500 http://raspbian.raspberrypi.org/raspbian stretch/non-free armhf Packages
 release o=Raspbian,a=stable,n=stretch,l=Raspbian,c=non-free,b=armhf
 origin raspbian.raspberrypi.org
500 http://raspbian.raspberrypi.org/raspbian stretch/contrib armhf Packages
 release o=Raspbian,a=stable,n=stretch,l=Raspbian,c=contrib,b=armhf
 origin raspbian.raspberrypi.org
500 http://raspbian.raspberrypi.org/raspbian stretch/main armhf Packages
 release o=Raspbian,a=stable,n=stretch,l=Raspbian,c=main,b=armhf
 origin raspbian.raspberrypi.org
Pinned packages:

Answer 1

Before even trying to install a new package, it is wise to run apt-get update and apt-get full-upgrade to ensure that your OS has been fully updated.

Once done, then try to install the new package.

Answer 2

Adding -oDebug::pkgProblemResolver to the command line may sched some light on the problem. Usually, this happens when you're trying to install packages on an old system, and those packages are no longer provided. raspbian.raspberrypi.org only provides packages for a couple of distros (stable / oldstable), packages for older distros are removed to save space.