tcpspray6: 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/tcpspray6 - from package: ndisc6

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

On Debian possible default full paths are:

/usr/bin/tcpspray6 - from package: ndisc6

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

Error in other languages:
tcpspray6: bevel nie gevind nie
tcpspray6: comandă negăsită
tcpspray6: comando não encontrado
tcpspray6: commande introuvable
tcpspray6: command not found
tcpspray6: käsku ei ole
tcpspray6: không tìm thấy lệnh
tcpspray6: komanda nerasta
tcpspray6: Kommando nicht gefunden.
tcpspray6: kommandot finns inte
tcpspray6: komut yok
tcpspray6: nie znaleziono polecenia
tcpspray6: níor aimsíodh an t-ordú
tcpspray6: no se encontró la orden
tcpspray6: no s'ha trobat l'ordre
tcpspray6: opdracht niet gevonden
tcpspray6: parancs nem található
tcpspray6: perintah tidak ditemukan
tcpspray6: príkaz nenájdený
tcpspray6: příkaz nenalezen
tcpspray6: команда не найдена
tcpspray6: командата не е открита
tcpspray6: 命令找不到
tcpspray6: コマンドが見つかりません