dhclient-script: 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:

/sbin/dhclient-script - from package: dhclient
/usr/share/rear/skel/default/bin/dhclient-script - from package: rear

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

On Debian possible default full paths are:

/sbin/dhclient-script - from package: isc-dhcp-client

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

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