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/prep - from package: loki

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

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