rdiffdir: 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 CentOS possible default full paths are:

/usr/bin/rdiffdir - from package: duplicity

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/rdiffdir - from package: duplicity

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

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