To use devices inside your nspawn container do:
Edit systemd-nspawn@<machine_name>.service
and add a line like the below
DeviceAllow="/dev/serial/by-id/usb-Roboteq_Motor_Controller_SDC2XXX-if00 rwm"
DeviceAllow=char-ttyACM rwm
DeviceAllow=char-usb_device rwm
DeviceAllow=ttyACM rwm
To see valid device types do: