ndisc6: 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/ndisc6 - from package: ndisc6

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

On Debian possible default full paths are:

/usr/bin/ndisc6 - from package: ndisc6

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

Error in other languages:
ndisc6: bevel nie gevind nie
ndisc6: comandă negăsită
ndisc6: comando não encontrado
ndisc6: commande introuvable
ndisc6: command not found
ndisc6: käsku ei ole
ndisc6: không tìm thấy lệnh
ndisc6: komanda nerasta
ndisc6: Kommando nicht gefunden.
ndisc6: kommandot finns inte
ndisc6: komut yok
ndisc6: nie znaleziono polecenia
ndisc6: níor aimsíodh an t-ordú
ndisc6: no se encontró la orden
ndisc6: no s'ha trobat l'ordre
ndisc6: opdracht niet gevonden
ndisc6: parancs nem található
ndisc6: perintah tidak ditemukan
ndisc6: príkaz nenájdený
ndisc6: příkaz nenalezen
ndisc6: команда не найдена
ndisc6: командата не е открита
ndisc6: 命令找不到
ndisc6: コマンドが見つかりません