denial6: 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/denial6 - from package: thc-ipv6

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

Error in other languages:
denial6: bevel nie gevind nie
denial6: comandă negăsită
denial6: comando não encontrado
denial6: commande introuvable
denial6: command not found
denial6: käsku ei ole
denial6: không tìm thấy lệnh
denial6: komanda nerasta
denial6: Kommando nicht gefunden.
denial6: kommandot finns inte
denial6: komut yok
denial6: nie znaleziono polecenia
denial6: níor aimsíodh an t-ordú
denial6: no se encontró la orden
denial6: no s'ha trobat l'ordre
denial6: opdracht niet gevonden
denial6: parancs nem található
denial6: perintah tidak ditemukan
denial6: príkaz nenájdený
denial6: příkaz nenalezen
denial6: команда не найдена
denial6: командата не е открита
denial6: 命令找不到
denial6: コマンドが見つかりません