w3af_gui: 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/w3af_gui - from package: w3af

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

Error in other languages:
w3af_gui: bevel nie gevind nie
w3af_gui: comandă negăsită
w3af_gui: comando não encontrado
w3af_gui: commande introuvable
w3af_gui: command not found
w3af_gui: käsku ei ole
w3af_gui: không tìm thấy lệnh
w3af_gui: komanda nerasta
w3af_gui: Kommando nicht gefunden.
w3af_gui: kommandot finns inte
w3af_gui: komut yok
w3af_gui: nie znaleziono polecenia
w3af_gui: níor aimsíodh an t-ordú
w3af_gui: no se encontró la orden
w3af_gui: no s'ha trobat l'ordre
w3af_gui: opdracht niet gevonden
w3af_gui: parancs nem található
w3af_gui: perintah tidak ditemukan
w3af_gui: príkaz nenájdený
w3af_gui: příkaz nenalezen
w3af_gui: команда не найдена
w3af_gui: командата не е открита
w3af_gui: 命令找不到
w3af_gui: コマンドが見つかりません