ni6: 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 Debian possible default full paths are:

/usr/sbin/ni6 - from package: ipv6toolkit

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

Error in other languages:
ni6: bevel nie gevind nie
ni6: comandă negăsită
ni6: comando não encontrado
ni6: commande introuvable
ni6: command not found
ni6: käsku ei ole
ni6: không tìm thấy lệnh
ni6: komanda nerasta
ni6: Kommando nicht gefunden.
ni6: kommandot finns inte
ni6: komut yok
ni6: nie znaleziono polecenia
ni6: níor aimsíodh an t-ordú
ni6: no se encontró la orden
ni6: no s'ha trobat l'ordre
ni6: opdracht niet gevonden
ni6: parancs nem található
ni6: perintah tidak ditemukan
ni6: príkaz nenájdený
ni6: příkaz nenalezen
ni6: команда не найдена
ni6: командата не е открита
ni6: 命令找不到
ni6: コマンドが見つかりません