flood_router26: 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_router26 - from package: thc-ipv6

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

Error in other languages:
flood_router26: bevel nie gevind nie
flood_router26: comandă negăsită
flood_router26: comando não encontrado
flood_router26: commande introuvable
flood_router26: command not found
flood_router26: käsku ei ole
flood_router26: không tìm thấy lệnh
flood_router26: komanda nerasta
flood_router26: Kommando nicht gefunden.
flood_router26: kommandot finns inte
flood_router26: komut yok
flood_router26: nie znaleziono polecenia
flood_router26: níor aimsíodh an t-ordú
flood_router26: no se encontró la orden
flood_router26: no s'ha trobat l'ordre
flood_router26: opdracht niet gevonden
flood_router26: parancs nem található
flood_router26: perintah tidak ditemukan
flood_router26: príkaz nenájdený
flood_router26: příkaz nenalezen
flood_router26: команда не найдена
flood_router26: командата не е открита
flood_router26: 命令找不到
flood_router26: コマンドが見つかりません