passive_discovery6: 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/passive_discovery6 - from package: thc-ipv6

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

Error in other languages:
passive_discovery6: bevel nie gevind nie
passive_discovery6: comandă negăsită
passive_discovery6: comando não encontrado
passive_discovery6: commande introuvable
passive_discovery6: command not found
passive_discovery6: käsku ei ole
passive_discovery6: không tìm thấy lệnh
passive_discovery6: komanda nerasta
passive_discovery6: Kommando nicht gefunden.
passive_discovery6: kommandot finns inte
passive_discovery6: komut yok
passive_discovery6: nie znaleziono polecenia
passive_discovery6: níor aimsíodh an t-ordú
passive_discovery6: no se encontró la orden
passive_discovery6: no s'ha trobat l'ordre
passive_discovery6: opdracht niet gevonden
passive_discovery6: parancs nem található
passive_discovery6: perintah tidak ditemukan
passive_discovery6: príkaz nenájdený
passive_discovery6: příkaz nenalezen
passive_discovery6: команда не найдена
passive_discovery6: командата не е открита
passive_discovery6: 命令找不到
passive_discovery6: コマンドが見つかりません