You may have to try a different way to get the MAC address. If you can’t log onto the device itself, maybe look at the MAC address table on the switch (if you know what port it’s attached to), or if it’s on the same subnet as your machine (or another machine you can log onto) use the ‘arp’ command after pinging it to show the mapping of IP address to MAC address.
Then once you have the MAC address you can manually configure the DHCP reservation.