I downloaded the Upwork desktop App on ubuntu 18.04. When I open it with Gdebi package installer I saw this error:

I tried to install gksu manually via sudo apt install gksu but I got this error

Building dependency tree
Reading state information... Done
Package gksu 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 'gksu' has no installation candidate

How can I install this software on ubuntu 18.04 (I download it yesturday so it is the most recent version on upwork website) ?

2 Answers

Introduction

As Jeremy Bicha wrote:

Posted on April 18, 2018 by Jeremy Bicha — 8 Comments
Today, gksu was removed from Ubuntu 18.04, four weeks after it was removed from Debian.

So theoretically speaking there are no packages on Bionic Beaver 18.04 LTS, depending on gksu. But this is not true:

$ apt-cache rdepends gksu
gksu
Reverse Depends: gexec wicd-gtk spacefm-gtk3 spacefm |menu
$ aptitude why gksu
i menu Suggests gksu | kde-runtime | ktsuss

As you see some software, which are distibuted by third-party may depend on gksu package. So you can proceed installing gksu with method below on your own risk.

How to fix?

Possible solution is to install version from previous (17.10, artful) release.

Warning: do not execute the commands below if unsure!

Short method (as @MK suggested)

wget
sudo apt install ./libgtop-2.0-10_2.32.0-1_amd64.deb
wget
sudo apt install ./libgksu2-0_2.0.13~pre1-6ubuntu8_amd64.deb
wget
sudo apt install ./gksu_2.0.2-9ubuntu1_amd64.deb

Test

Test gksu (should work on Xorg-sessions)

gksu-properties # check that it has "Authentication mode" to "sudo"
gksu date
gksudo date

Notes

After installation the following packages will be marked as obsolete (locally-installed): gksu, libgksu2-0. But they will work as expected.

Mixed repositories

Adding the artful repositories to a bionic system is an absolutely horrible idea. I cannot stress enough how much suffering the notion, alone, of running with mixed repositories will bring. There shall be misery and gnashing of teeth to the end of the age and beyond.

Masochists, please at least use protection (Apt Pinning).

Better ideas

Wait for new releases that do not depend on outdated packages. (Patience is a virtue that will prevent embarrassing visits to the Help Center with conf-files, or worse, stuck where they don't belong.)
Rebuild packages without unnecessary dependencies.
Find PPAs with the packages you want.
Put a wrapper -script- around pkexec.
Faux play -with faux packages- and equivs.

Notes