waterfall: 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/waterfall - from package: gwaterfall

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

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