pb-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:

/usr/sbin/pb-udhcpc - from package: petitboot

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

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