Port not found when flashing the application to nRF52840-dongle

I was trying someof the examples RIOT has. The problem is that RIOT’s flash uses /dev/ttyACM0 by default and when I connect the nRF52840-dongle it’s assigned a port /dev/ttyACM1.

This results in failing to flash!! Has anyone idea how to specify port while flashing? like with

make flash BOARD= nrf52840-dongle -p /dev/ttyACM1

-p has a very diffent meaning with make (see make(1) manual page). You need to provide it as an environment variable to make as well:

make flash BOARD=nrf52840-dongle PORT=/dev/ttyACM1

Note, that there is no space after the =, otherwise your shell will parse it as another argument. See also the Getting started guide, Building and executing an example.