tcptraceroute6: 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/tcptraceroute6 - from package: ndisc6

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

On Debian possible default full paths are:

/usr/bin/tcptraceroute6 - from package: ndisc6

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

Error in other languages:
tcptraceroute6: bevel nie gevind nie
tcptraceroute6: comandă negăsită
tcptraceroute6: comando não encontrado
tcptraceroute6: commande introuvable
tcptraceroute6: command not found
tcptraceroute6: käsku ei ole
tcptraceroute6: không tìm thấy lệnh
tcptraceroute6: komanda nerasta
tcptraceroute6: Kommando nicht gefunden.
tcptraceroute6: kommandot finns inte
tcptraceroute6: komut yok
tcptraceroute6: nie znaleziono polecenia
tcptraceroute6: níor aimsíodh an t-ordú
tcptraceroute6: no se encontró la orden
tcptraceroute6: no s'ha trobat l'ordre
tcptraceroute6: opdracht niet gevonden
tcptraceroute6: parancs nem található
tcptraceroute6: perintah tidak ditemukan
tcptraceroute6: príkaz nenájdený
tcptraceroute6: příkaz nenalezen
tcptraceroute6: команда не найдена
tcptraceroute6: командата не е открита
tcptraceroute6: 命令找不到
tcptraceroute6: コマンドが見つかりません