pkgdiff: 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/pkgdiff - from package: pkgdiff

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

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