umount.nfs: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/sbin/umount.nfs - from package: nfs-utils

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/sbin/umount.nfs - from package: nfs-common

You may install the required package with command apt-get install {package}

Error in other languages:
umount.nfs: bevel nie gevind nie
umount.nfs: comandă negăsită
umount.nfs: comando não encontrado
umount.nfs: commande introuvable
umount.nfs: command not found
umount.nfs: käsku ei ole
umount.nfs: không tìm thấy lệnh
umount.nfs: komanda nerasta
umount.nfs: Kommando nicht gefunden.
umount.nfs: kommandot finns inte
umount.nfs: komut yok
umount.nfs: nie znaleziono polecenia
umount.nfs: níor aimsíodh an t-ordú
umount.nfs: no se encontró la orden
umount.nfs: no s'ha trobat l'ordre
umount.nfs: opdracht niet gevonden
umount.nfs: parancs nem található
umount.nfs: perintah tidak ditemukan
umount.nfs: príkaz nenájdený
umount.nfs: příkaz nenalezen
umount.nfs: команда не найдена
umount.nfs: командата не е открита
umount.nfs: 命令找不到
umount.nfs: コマンドが見つかりません