rpmdep: 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/rpmdep - from package: rpmorphan

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

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