update-initramfs: 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-initramfs - from package: initramfs-tools
/usr/sbin/update-initramfs - from package: live-tools

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

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