kill_router6: 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/kill_router6 - from package: thc-ipv6

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

Error in other languages:
kill_router6: bevel nie gevind nie
kill_router6: comandă negăsită
kill_router6: comando não encontrado
kill_router6: commande introuvable
kill_router6: command not found
kill_router6: käsku ei ole
kill_router6: không tìm thấy lệnh
kill_router6: komanda nerasta
kill_router6: Kommando nicht gefunden.
kill_router6: kommandot finns inte
kill_router6: komut yok
kill_router6: nie znaleziono polecenia
kill_router6: níor aimsíodh an t-ordú
kill_router6: no se encontró la orden
kill_router6: no s'ha trobat l'ordre
kill_router6: opdracht niet gevonden
kill_router6: parancs nem található
kill_router6: perintah tidak ditemukan
kill_router6: príkaz nenájdený
kill_router6: příkaz nenalezen
kill_router6: команда не найдена
kill_router6: командата не е открита
kill_router6: 命令找不到
kill_router6: コマンドが見つかりません