rdiff-backup: 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/rdiff-backup - from package: rdiff-backup

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

On Debian possible default full paths are:

/usr/bin/rdiff-backup - from package: rdiff-backup

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

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