Kernel don`t have nfsd support.
gb-brix-3000# uname -a
Linux gb-brix-3000 4.4.5-amd64-vyos #1 SMP Fri Mar 11 06:56:45 EST 2016 x86_64 GNU/Linux
gb-brix-3000# systemctl status nfs-kernel-server
nfs-kernel-server[6698]: Not starting NFS kernel daemon: no support in current kernel. ... (warning).
Apr 19 17:18:56 gb-brix-3000 nfs-kernel-server[6725]: Starting NFS kernel daemon: nfsdrpc.nfsd: Unable to access /proc/fs/nfsd errno 2 (No such file or directory).
Apr 19 17:18:56 gb-brix-3000 nfs-kernel-server[6725]: Please try, as root, 'mount -t nfsd nfsd /proc/fs/nfsd' and then restart rpc.nfsd to correct the problem
Apr 19 17:18:56 gb-brix-3000 nfs-kernel-server[6725]: failed!
Apr 19 17:18:56 gb-brix-3000 systemd[1]: nfs-kernel-server.service: control process exited, code=exited status=1
Apr 19 17:18:56 gb-brix-3000 systemd[1]: Failed to start LSB: Kernel NFS server support.
Apr 19 17:18:56 gb-brix-3000 systemd[1]: Unit nfs-kernel-server.service entered failed state.
gb-brix-3000# cat config-4.4.5-amd64-vyos | grep NFSD
#CONFIG_NFSD is not set
gb-brix-3000# cat config-4.4.6-amd64-vyos | grep NFSD
#CONFIG_NFSD is not set