nufw: 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/sbin/nufw - from package: nufw

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

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