rhgb-client: 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/rhgb-client - from package: plymouth

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

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