Standard package missing

Question

I am trying to install httpie:

» sudo apt update
Hit:1 http://archive.ubuntu.com/ubuntu bionic InRelease
Hit:2 http://archive.ubuntu.com/ubuntu bionic-security InRelease
Hit:3 http://archive.ubuntu.com/ubuntu bionic-updates InRelease
Reading package lists... Done                     
Building dependency tree       
Reading state information... Done
All packages are up to date.

» sudo apt install httpie
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Package httpie 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 'httpie' has no installation candidate

But the package is listed

This happens for other packages also:

E: Unable to locate package git-annex
E: Package 'httpie' has no installation candidate
E: Unable to locate package jshon
E: Unable to locate package python-pip
E: Unable to locate package tree

But they exist too:

• git-annex
• jshon
• python-pip
• tree

What is going on?

This is my sources.list:

» cat /etc/apt/sources.list
deb http://archive.ubuntu.com/ubuntu bionic main
deb http://archive.ubuntu.com/ubuntu bionic-security main
deb http://archive.ubuntu.com/ubuntu bionic-updates main

And this is my system:

» lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 18.04.1 LTS
Release:        18.04
Codename:       bionic

EDIT

This is an ubuntu server system, no gui.