After running apt-get, I got:
proxmox3:~/backup# apt-get install nfs-common
Reading package lists... Done
Building dependency tree... Done
nfs-common is already the newest version.
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
I must say, I am trying to set-up the Proxmox server as an NFS client.
Thanks for your kind help.