Selecting the correct board from the tools menu is an important step that needs to be completed before you can upload your code. If this option is greyed out, there are a few things that you can do to resolve this issue.
1. Make sure that the board is working and corrected properly
The 'Port' option can be greyed out if no devices are connected to the computer. It's a good idea to check the Brain Board is properly connected to the computer. Ensure that the blue USB cable has been inserted fully both into the board and the USB port of your computer.
If the board is working, you will see lights on the board activate when it is activated to power. If these lights do not turn on and you are sure that the board is fully connected, try another computer or device that supplies a low-power USB port.
2. Connect the Brain Board to another port on your computer
The port you are using on the computer could be having troubles. Try connecting the board to another USB port.
After these steps, check to see if the 'Port' menu has re-enabled
You can check to see if these steps have worked by going to the menu at the top of your window and selecting Tools > Port > Arduino/Genuino Uno.