The binding also uses the “does not exist” message for cases when it simply doesn’t have permission to access the device. On my system (raspbian), ls -l /dev/ttyACM0 gives this output:
crw-rw---- 1 root dialout 166, 0 Oct 16 10:18 /dev/ttyACM0
which means that only members of the dialout group can access the device. You can add the openhab user to the dialout group with the command: adduser openhab dialout . After that, you’ll need to restart the OpenHAB service so it’ll see its new group membership.