dhcp-helper: 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 Debian possible default full paths are:

/usr/sbin/dhcp-helper - from package: dhcp-helper

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

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