Because there's no service with such a name on Ubuntu. Try
Code:
systemctl list-unit-files -t service
and see for yourself.
Code:
$ for p in kernel-server ganesha common; do apt-file list nfs-$p|grep service$;done
nfs-kernel-server: /lib/systemd/system/nfs-blkmap.service
nfs-kernel-server: /lib/systemd/system/nfs-kernel-server.service
nfs-kernel-server: /lib/systemd/system/nfs-mountd.service
nfs-kernel-server: /lib/systemd/system/nfs-server.service
nfs-ganesha: /lib/systemd/system/nfs-ganesha-config.service
nfs-ganesha: /lib/systemd/system/nfs-ganesha-lock.service
nfs-ganesha: /lib/systemd/system/nfs-ganesha.service
nfs-common: /lib/systemd/system/auth-rpcgss-module.service
nfs-common: /lib/systemd/system/nfs-common.service
nfs-common: /lib/systemd/system/nfs-config.service
nfs-common: /lib/systemd/system/nfs-idmapd.service
nfs-common: /lib/systemd/system/nfs-utils.service
nfs-common: /lib/systemd/system/rpc-gssd.service
nfs-common: /lib/systemd/system/rpc-statd-notify.service
nfs-common: /lib/systemd/system/rpc-statd.service
nfs-common: /lib/systemd/system/rpc-svcgssd.service