Either platform file should be fine. The syntax within you make target defines which of the two platform files are accessed.
It's a generic error which occurs whenever the HCI UART (the UART on the device designed to be a programming interface only) is in application mode. You will want to open the case and press the reset button (WICED Sense 1) or simply press the reset button on WICED Sense 2 (external reset was added to this device). Essentially, each time the device is programmed, it then goes into an application mode and the HCI UART is non-responsive to additional programming until it is put back into programming mode. There are dozens of threads here on the forum that reference this as it is a common area of frustration.
The boot sequence is described here and in a few other threads: BCM2073XS Boot Sequence: Can't download to board (device not found) and here is another How to boot up sequence?