Unable to upload to STM32 Thing Plus

If it’s like the STM32 MicroMod board, it should definitely be visible as a DFU device when in bootloader mode. On Linux, I get this entry in the output from lsusb:

Bus 001 Device 007: ID 0483:df11 STMicroelectronics STM Device in DFU Mode

You shouldn’t get a COM port for the device while in boot mode, as far as I can tell.