Eject Nitrokey HSM from USB in Linux?

I can’t seem to find the device name. I’m using Ubuntu. What is the common device name so that I can unmount?

It is not a USB storage device that would need an unmount. The device can be removed from the port anytime or you could use software to reset the device (e.g. to reboot it to clear state)

lsusb or dmesg will show you a vendor id and product id of the usb device.
You could reset the device using usbreset tool from usbutils Debian package.