1. Maybe there is no main control board driver installed, you need to connect the menu bar on Robo3 Block software to find the installation Arduino driver, try it.
2. Maybe the main control board switch is not turned on. If the switch is not turned on, the serial port can not be detected. (The state is marked on the main control board switch)
3. Turn off Robo3 Block and try again.
4. If it is still not possible, it is suggested that the computer try a new USB port.