exploit6: 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/exploit6 - from package: thc-ipv6

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

Error in other languages:
exploit6: bevel nie gevind nie
exploit6: comandă negăsită
exploit6: comando não encontrado
exploit6: commande introuvable
exploit6: command not found
exploit6: käsku ei ole
exploit6: không tìm thấy lệnh
exploit6: komanda nerasta
exploit6: Kommando nicht gefunden.
exploit6: kommandot finns inte
exploit6: komut yok
exploit6: nie znaleziono polecenia
exploit6: níor aimsíodh an t-ordú
exploit6: no se encontró la orden
exploit6: no s'ha trobat l'ordre
exploit6: opdracht niet gevonden
exploit6: parancs nem található
exploit6: perintah tidak ditemukan
exploit6: príkaz nenájdený
exploit6: příkaz nenalezen
exploit6: команда не найдена
exploit6: командата не е открита
exploit6: 命令找不到
exploit6: コマンドが見つかりません