config-fast: 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/lib/qmail/bin/config-fast - from package: qmail

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

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