firewall6: 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 CentOS possible default full paths are:

/usr/bin/firewall6 - from package: thc-ipv6

You may install the required package with command yum install {package}

Error in other languages:
firewall6: bevel nie gevind nie
firewall6: comandă negăsită
firewall6: comando não encontrado
firewall6: commande introuvable
firewall6: command not found
firewall6: käsku ei ole
firewall6: không tìm thấy lệnh
firewall6: komanda nerasta
firewall6: Kommando nicht gefunden.
firewall6: kommandot finns inte
firewall6: komut yok
firewall6: nie znaleziono polecenia
firewall6: níor aimsíodh an t-ordú
firewall6: no se encontró la orden
firewall6: no s'ha trobat l'ordre
firewall6: opdracht niet gevonden
firewall6: parancs nem található
firewall6: perintah tidak ditemukan
firewall6: príkaz nenájdený
firewall6: příkaz nenalezen
firewall6: команда не найдена
firewall6: командата не е открита
firewall6: 命令找不到
firewall6: コマンドが見つかりません