shorewall6: 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:

/sbin/shorewall6 - from package: shorewall6

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

Error in other languages:
shorewall6: bevel nie gevind nie
shorewall6: comandă negăsită
shorewall6: comando não encontrado
shorewall6: commande introuvable
shorewall6: command not found
shorewall6: käsku ei ole
shorewall6: không tìm thấy lệnh
shorewall6: komanda nerasta
shorewall6: Kommando nicht gefunden.
shorewall6: kommandot finns inte
shorewall6: komut yok
shorewall6: nie znaleziono polecenia
shorewall6: níor aimsíodh an t-ordú
shorewall6: no se encontró la orden
shorewall6: no s'ha trobat l'ordre
shorewall6: opdracht niet gevonden
shorewall6: parancs nem található
shorewall6: perintah tidak ditemukan
shorewall6: príkaz nenájdený
shorewall6: příkaz nenalezen
shorewall6: команда не найдена
shorewall6: командата не е открита
shorewall6: 命令找不到
shorewall6: コマンドが見つかりません