rfl: 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/share/doc/mh-book/examples/mh/bin/rfl - from package: mh-book

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

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