If you’re trying to figure out why you are getting NFS errors despite having NFS set up properly on the host machine and client VM, this may be the answer. With the VM powered off, run at the Proxmox terminal:
vzctl set [VM ID] --features "nfs:on" --save
The next time the VM is powered up NFS should be enabled and you should be able to mount shares correctly. An example:
vzctl set 104 --features "nfs:on" --save