downtime: 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/downtime - from package: downtimed

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

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