umount.crypt_LUKS: 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 Debian possible default full paths are:

/sbin/umount.crypt_LUKS - from package: libpam-mount

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

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