Hi Park,
Without a ground connection between the CC260STK board and the DRV8830 board there isn't a complete circuit for the I2C communication.
Unplug the USB power from the CC260STK board.
In your picture supplied the Blue wire connected to the CC260STK should be the GND connection for that board. Try connecting the blue wire to the black test point labeled GND on the bottom front of the EVM.
Make sure the motor is connected to the DRV8830 EVM.
After making this connection power on the DRV8830 EVM then plug in the USB cable to the CC260STK board.
Without a ground connection between the CC260STK board and the DRV8830 board there isn't a complete circuit for the I2C communication.
Unplug the USB power from the CC260STK board.
In your picture supplied the Blue wire connected to the CC260STK should be the GND connection for that board. Try connecting the blue wire to the black test point labeled GND on the bottom front of the EVM.
Make sure the motor is connected to the DRV8830 EVM.
After making this connection power on the DRV8830 EVM then plug in the USB cable to the CC260STK board.