Due to the improvements in Ubuntu, they got rid of libgnome2, which we use for our GTK client. This means that if you try to install Plastic's GUI it is going to prompt an error and fail.
However, there is a way to work around this if you want to get your hands dirty.
What we are going to do is to add the library from a previous version, Ubuntu 18 - Bionic.
To do so, we need to create the "bionic-oldlibs.list" file under "/etc/apt/sources.list.d".
Inside that file paste these two lines:
deb http://us.archive.ubuntu.com/ubuntu/ bionic main restricted
deb http://us.archive.ubuntu.com/ubuntu/ bionic universe
Those are the latest long-term Ubuntu repositories with libgnome2.
Once you have it, performing an "apt update" and later on the usual Plastic installation would get you the client without further problem.
Hope you enjoy it!