udhcpc: 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:

/sbin/udhcpc - from package: udhcpc

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

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