rpmdiff: 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/rpmdiff - from package: rpmlint

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

On Debian possible default full paths are:

/usr/bin/rpmdiff - from package: rpmlint

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

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