"Package synaptic is not available" in Ubuntu 16.04

I can't install synaptic package manager in Ubuntu 16.04.
I do this:

sudo apt-get update
sudo apt-get upgrade
sudo apt-get install synaptic

And I see

Package synaptic 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 'synaptic' has no installation candidate

What can I do?

The output of sudo apt update command:

Hit:1 http://archive.canonical.com xenial InRelease
Hit:2 http://archive.canonical.com/ubuntu xenial InRelease
Reading package lists... Done
Building dependency tree
Reading state information... Done
All packages are up to date.