upgrade-system: 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/upgrade-system - from package: upgrade-system

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

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