flood_dhcpc6: 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/flood_dhcpc6 - from package: thc-ipv6

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

Error in other languages:
flood_dhcpc6: bevel nie gevind nie
flood_dhcpc6: comandă negăsită
flood_dhcpc6: comando não encontrado
flood_dhcpc6: commande introuvable
flood_dhcpc6: command not found
flood_dhcpc6: käsku ei ole
flood_dhcpc6: không tìm thấy lệnh
flood_dhcpc6: komanda nerasta
flood_dhcpc6: Kommando nicht gefunden.
flood_dhcpc6: kommandot finns inte
flood_dhcpc6: komut yok
flood_dhcpc6: nie znaleziono polecenia
flood_dhcpc6: níor aimsíodh an t-ordú
flood_dhcpc6: no se encontró la orden
flood_dhcpc6: no s'ha trobat l'ordre
flood_dhcpc6: opdracht niet gevonden
flood_dhcpc6: parancs nem található
flood_dhcpc6: perintah tidak ditemukan
flood_dhcpc6: príkaz nenájdený
flood_dhcpc6: příkaz nenalezen
flood_dhcpc6: команда не найдена
flood_dhcpc6: командата не е открита
flood_dhcpc6: 命令找不到
flood_dhcpc6: コマンドが見つかりません