Cannot Serial/Screen into things error "Cannot open line '/dev/ttyUSBtoSerial for r/w: resource busy" "Sorry, could not find a PTY"
Job #1 - is to determine if the USB device is being recognized in any form, you can do that by looking for the USB VID and PID (described below).  When you plug the cable in, you should see the device appear in the list, and go away when you unplug.

The VID number will tell you the ACTUAL chip type, ie: FTDI, SiLAB, Prolific, or Microchip there are many chips in these cables. Your comments imply you believe this is an FTDI cable, this will confirm. Otherwise you are going after the WRONG driver.

Job #2 - Using the VENDOR ID - determine if the driver is loaded or not, or is present. For example once you confirm it is an FTDI cable, then the existing Apple provided driver should be present, same with SiLABs (apple provides this driver).

To do the above:

First verify that this is an FTDI cable, there are many other chips (Microchip, SiLab, and Prolific) - some vendors (example: Adafruit, and others) have been known to change the chip inside with different cable builds.

The GUI method on macOS is: 
 -> About this Mac -> System Report, under hardware click on USB.

There are multiple "root hubs" - my MacPro has a total of 5 (some hubs are built in and hidden inside your Mac and not exposed externally)

Plug &amp; Unplug your device, in the menubar of System Information -> File -> Refresh (CMD-R) to update the list displayed.

Look for the "Vendor ID (VID)" (who makes the USB device) and "Product ID (PID)".
for example FTDI = VID: 0403, PID = 6010 (for the FTD2232H, the PID might vary depending on the chip).

As an alternative you can open Terminal and type: ioreg -p IOUSB -l -w 0 and get a lot of of details.

You can do the same thing with Windows in the Device Manager.
On Linux, you can use lsusb to print the USB devices.

This will verify that your device is even "half recognized".

Part 1 - is that the OS can find the VID and PID

Part 2 - is the driver for that USB device

Note that FTDI has a built in driver in Catalina, no external driver required.