By default, the nRF52840-DK operates at 3V. The SCuM board operates at 1.8V and you will destroy it if you connect it to the nRF52840-DK operating at 3V.
You need to load the
scum-programmer.hex firmware onto the nRF52840-DK before connecting the SCuM board. This firmware switch the nRF52840-DK to 1.8V.