rdisc6: 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/sbin/rdisc6 - from package: ndisc6

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

On Debian possible default full paths are:

/bin/rdisc6 - from package: ndisc6

You may install the required package with command apt-get install {package}

Error in other languages:
rdisc6: bevel nie gevind nie
rdisc6: comandă negăsită
rdisc6: comando não encontrado
rdisc6: commande introuvable
rdisc6: command not found
rdisc6: käsku ei ole
rdisc6: không tìm thấy lệnh
rdisc6: komanda nerasta
rdisc6: Kommando nicht gefunden.
rdisc6: kommandot finns inte
rdisc6: komut yok
rdisc6: nie znaleziono polecenia
rdisc6: níor aimsíodh an t-ordú
rdisc6: no se encontró la orden
rdisc6: no s'ha trobat l'ordre
rdisc6: opdracht niet gevonden
rdisc6: parancs nem található
rdisc6: perintah tidak ditemukan
rdisc6: príkaz nenájdený
rdisc6: příkaz nenalezen
rdisc6: команда не найдена
rdisc6: командата не е открита
rdisc6: 命令找不到
rdisc6: コマンドが見つかりません