It would appear the adapter is not communicating with the scanner.
That's why he asked the question.
It to me is either a bad cable or driver issue.
If you have another cable try that. I had one go bad on me and suggested to others with similar questions in the forums the same and that it indeed were bad cable issues. Cable that went bad was the one from Uniden and not the converter cable.
Make certain to use the driver(s) for your cable from the cable manufacturer and hopefully a driver written for your windows version ideally. If windows 10, no drivers available, use windows 8, if none there use windows 7 drivers.
In the link you gave, on middle right side are the drivers link. Here it is,
http://sgcdn.startech.com/005329/media/sets/Prolific-PL-2303_Drivers/Prolific_PL2303.zip
If using those don't help, try righclicking the setup.exe file and select 'Run as administrator'
Always make certain when you select your Virtual Control, click the button that shows the 'COM Port Setup'.
This brings up another window, then select the 'Start Auto' button.
Windows likes to change ports many times.
Another thing is to set your 996XT in the 'Settings' to 115200 baud.
Others in the past mention trying lower baud rates, but that normally hasn't helped. I have helped many others with programming XT models and haven't had any problems due to baud rates.