Hi Terry,
Thank you for giving that a try. To clarify, doe the “Security & Privacy > General” panel still show the option to allow the Prolific software or did this go away after selecting to Allow?
Please also let me know if the setting to “Allow apps downloaded from:” is set to “App Store and identified developers”, this is located on the “Security & Privacy > General” panel.
While I have not been able to reproduce a similar error I have had other customer’s report similar issues where macOS would not allow the drivers to start even after reinstalling the drivers multiple times, updating the system and going through the steps to allow Prolific software. Disabling System Integrity Protection and reinstalling the driver using the instructions in the zip file, then verifying the serial adapter is functional, then re-enabling System Integrity Protection then allowed the software to load correctly at boot. If you would like to give this a try let me know and I can provide some additional instructions, please note, disabling System Integrity Protection may leave the system vulnerable to malicious software until System Integrity Protection is re-enabled.
Thank you,
Pat