Thanks for reaching out to us, I am sorry this is not working as expected and I am happy to help.
Unfortunately, disappearing /dev/tty.usbserial on MacOS has been seen on forums and troubleshooting pages for devices like Arduino and other common USB-to-Serial devices used with MacOS. The root of the problem appears to be devices which open and close serial communications quickly causes MacOS to stop detecting the USB port after the serial device has been disconnected or the connection closed.
The first step in troubleshooting this problem is to update the drivers, please download and install the latest drivers for the Prolific PL2303HX chipset here ( https://plugable.com/drivers/prolific ). Please note: MacOS has added additional security within Gatekeeper requiring the user to specifically allow the installation of new drivers. We have a blog post here detailing how to work with Gatekeeper to install these, and other, drivers ( https://plugable.com/2017/10/27/how-t… ).
After downloading and installing the drivers, please restart the system even if not prompted to do so.