tcpspray.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 Debian possible default full paths are:

/usr/bin/tcpspray.ndisc6 - from package: ndisc6

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

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