unattended-upgrades: 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/bin/unattended-upgrades - from package: unattended-upgrades

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

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