traceroute6: 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:

/bin/traceroute6 - from package: traceroute

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

Error in other languages:
traceroute6: bevel nie gevind nie
traceroute6: comandă negăsită
traceroute6: comando não encontrado
traceroute6: commande introuvable
traceroute6: command not found
traceroute6: käsku ei ole
traceroute6: không tìm thấy lệnh
traceroute6: komanda nerasta
traceroute6: Kommando nicht gefunden.
traceroute6: kommandot finns inte
traceroute6: komut yok
traceroute6: nie znaleziono polecenia
traceroute6: níor aimsíodh an t-ordú
traceroute6: no se encontró la orden
traceroute6: no s'ha trobat l'ordre
traceroute6: opdracht niet gevonden
traceroute6: parancs nem található
traceroute6: perintah tidak ditemukan
traceroute6: príkaz nenájdený
traceroute6: příkaz nenalezen
traceroute6: команда не найдена
traceroute6: командата не е открита
traceroute6: 命令找不到
traceroute6: コマンドが見つかりません