USB to Serial on macOS Sierra not working
Install libusb >1.0 (actual 1.0.20) from homebrew or MacPorts, since this one provides access for controlling data transfer to and from USB devices in UNIX/non-UNIX systems, without the need for kernel-mode drivers.

As for a free application, try CoolTerm.

I used this combination with Cortex-M4 microcontrollers.