Despite following the setup guide, I suspect this is a problem with your NFS settings on your NAS. Can you do some screen shots of your settings please?
Also, if you are able to ssh in to your RPi:
can you paste the result here of typing the command:
sudo showmount -e 192.168.2.178.21
Lastly, please try entering the IP address as 192.168.2.178.21 when you try and add a drive in the Volumio UI.