unattended-upgrade: command not found

Have You tried accessing the binary with the full path?

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH

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-upgrade - from package: unattended-upgrades

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