Your issue could be due to mismatched baud or a cable problem.
In the radio menu 56, set the baud rate to 57600 then set the RigPi baud rate to the same and try it again.
The cable that I tested with is a FTDI cable that I got several years ago on E-bay and I do not have any experience with Prolific cables.
I have never needed to install drivers on the RigPi for any of the half dozen cables that are in my shack.
73 - Mark - K8MHW
'A RigPi Enthusiast!' - I am not otherwise affiliated with MFJ or RigPi