flood_mld26: 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_mld26 - from package: thc-ipv6

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

Error in other languages:
flood_mld26: bevel nie gevind nie
flood_mld26: comandă negăsită
flood_mld26: comando não encontrado
flood_mld26: commande introuvable
flood_mld26: command not found
flood_mld26: käsku ei ole
flood_mld26: không tìm thấy lệnh
flood_mld26: komanda nerasta
flood_mld26: Kommando nicht gefunden.
flood_mld26: kommandot finns inte
flood_mld26: komut yok
flood_mld26: nie znaleziono polecenia
flood_mld26: níor aimsíodh an t-ordú
flood_mld26: no se encontró la orden
flood_mld26: no s'ha trobat l'ordre
flood_mld26: opdracht niet gevonden
flood_mld26: parancs nem található
flood_mld26: perintah tidak ditemukan
flood_mld26: príkaz nenájdený
flood_mld26: příkaz nenalezen
flood_mld26: команда не найдена
flood_mld26: командата не е открита
flood_mld26: 命令找不到
flood_mld26: コマンドが見つかりません