dhclient: 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 - from package: dhclient

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

On Debian possible default full paths are:

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

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

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