Hi Terry,
Thanks for double checking this! For disabling SIP, I do recommend removing the Prolific drivers first, instructions are provided in the PDF file in the driver zip file ( near the end of the file ) which includes deleting the driver files from the terminal. After the drivers are removed, please restart the system before disabling SIP, and with SIP disabled reinstall the driver package and test the USB serial adapter before re-enabling SIP to verify it is working with the system.
Please let me know if this helps to get the driver functional on the system.
Thank you,
Pat