rdup-simple: 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/rdup-simple - from package: rdup

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

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