repodiffer: 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/repodiffer - from package: reposurgeon

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

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