It is easy enough to control the Next button, so it becomes disabled if none or multiple packages were selected for installation, but as I mentioned, this is not how this control is supposed to be used in interfaces. For single choice selections, the radiobutton control should be used. I suggest that you use a Radio Buttons screen, or a Custom screen and list your options there. It will be clear that only one option can be selected.
In the attached project file, you can see how in control the state of the Next button of the Select Packages screen in the On Ctrl Message event script.
You can also see in the On Preload event script of the Radio Buttons screen how to extract the package names and descriptions, and use them for setting the captions of the radiobutton controls. The number of radiobuttons is adjusted according to the number of packages. In the On Ctrl Message event script of this screen, I update the state of the packages, so if you jump back to the previous screen, you will see the latest selection reflected there (what was set via radiobutton). You can jump back and forth between the two screens and should see the change done in the previous screen.
Ulrich
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