umount: 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:

/bin/umount - from package: util-linux-ng

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

On Debian possible default full paths are:

/bin/umount - from package: mount
/usr/lib/klibc/bin/umount - from package: klibc-utils

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

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