USB to TTL communication problem
You may well have gotten bit by a fake  Prolific 2303 chip.

The Prolific driver download site has a warning and also says how to detect the fakes.

Basically, check the Windows Device Manager.  If your Serial/USB device shows a yellow warning triangle and has error code 10, then you have a fake.  It will not work with newer Prolific drivers.

You might find an old Prolific driver that will work with your device.  There are lots of sites on the internet discussing this, and lots of advice on finding old drivers.

You might also just save yourself the time rooting around the internet and buy or order an adapter from someplace reliable so that you don't get burned by another fake and don't have to worry about the drivers.