mkinitramfs: 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/mkinitramfs - from package: initramfs-tools

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

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