update-grub: 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:

/usr/sbin/update-grub - from package: grub2-common
/usr/sbin/update-grub - from package: grub-legacy

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

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