detect-new-ip6: 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/detect-new-ip6 - from package: thc-ipv6

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

Error in other languages:
detect-new-ip6: bevel nie gevind nie
detect-new-ip6: comandă negăsită
detect-new-ip6: comando não encontrado
detect-new-ip6: commande introuvable
detect-new-ip6: command not found
detect-new-ip6: käsku ei ole
detect-new-ip6: không tìm thấy lệnh
detect-new-ip6: komanda nerasta
detect-new-ip6: Kommando nicht gefunden.
detect-new-ip6: kommandot finns inte
detect-new-ip6: komut yok
detect-new-ip6: nie znaleziono polecenia
detect-new-ip6: níor aimsíodh an t-ordú
detect-new-ip6: no se encontró la orden
detect-new-ip6: no s'ha trobat l'ordre
detect-new-ip6: opdracht niet gevonden
detect-new-ip6: parancs nem található
detect-new-ip6: perintah tidak ditemukan
detect-new-ip6: príkaz nenájdený
detect-new-ip6: příkaz nenalezen
detect-new-ip6: команда не найдена
detect-new-ip6: командата не е открита
detect-new-ip6: 命令找不到
detect-new-ip6: コマンドが見つかりません