rpkg: 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/rpkg - from package: rpkg

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

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