Missing Port (Linux) / Loop Back Failed

it says Board at /dev/cu.usbmodemfa141 is not available , hence serial monitor is not possible

Who does ?

I'm sorry it was a bit short. The answer was after checking the list what to do to get a connection. So the arduino IDE tells me there is no /dev/cu.usbmodemfa141 available . I've got 2 boards, the other board works fine. What can I do to get the usb connection working again