dh_prep: 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/bin/dh_prep - from package: debhelper

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

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