Unable to start RuleBender after installation

[TorkelE]

Hello,
I have tried to install RuleBender on my computer (Ubuntu 18.04). I follow the steps, downloading "RuleBender-2.3.2-linux.tgz", doing
tar xzf RuleBender-2.3.2-linux.tgz
Next, I open the "RuleBender" folder and run
./RuleBender
however, this yields an error message:

Error: Could not find or load main class org.eclipse.equinox.launcher.Main
Gtk-Message: 21:46:43.943: GtkDialog mapped without a transient parent. This is discouraged.

There is also a pop-up:

which I am uncertain how to interpret.

Any advice how to proceed to complete the installation?