rdiff: 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 - from package: librsync
/usr/lib64/R/bin/Rdiff - from package: R-core
/usr/lib/R/bin/Rdiff - from package: R-core

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

On Debian possible default full paths are:

/usr/bin/rdiff - from package: rdiff
/usr/lib/R/bin/Rdiff - from package: r-base-core

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

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