repodiff: 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/repodiff - from package: yum-utils

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

On Debian possible default full paths are:

/usr/bin/repodiff - from package: yum-utils

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

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