tcp6: 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/tcp6 - from package: ipv6toolkit

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

Error in other languages:
tcp6: bevel nie gevind nie
tcp6: comandă negăsită
tcp6: comando não encontrado
tcp6: commande introuvable
tcp6: command not found
tcp6: käsku ei ole
tcp6: không tìm thấy lệnh
tcp6: komanda nerasta
tcp6: Kommando nicht gefunden.
tcp6: kommandot finns inte
tcp6: komut yok
tcp6: nie znaleziono polecenia
tcp6: níor aimsíodh an t-ordú
tcp6: no se encontró la orden
tcp6: no s'ha trobat l'ordre
tcp6: opdracht niet gevonden
tcp6: parancs nem található
tcp6: perintah tidak ditemukan
tcp6: príkaz nenájdený
tcp6: příkaz nenalezen
tcp6: команда не найдена
tcp6: командата не е открита
tcp6: 命令找不到
tcp6: コマンドが見つかりません