If you're running the commands from the console or SSH then you don't need to specify the server. If you're using vSphere CLI remotely then you will need to add "--server [ADDRESS OF ESXI HOST]"
Enable maintenance mode
You may also do this using the vSphere client or the vSphere web client.
Make sure to shutdown or vMotion all of your running VMs as maintenance mode disables all VM operations.
esxcli system maintenanceMode set --enable true
Disable maintenance modeYou may also do thi