trace6: 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/trace6 - from package: thc-ipv6

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

Error in other languages:
trace6: bevel nie gevind nie
trace6: comandă negăsită
trace6: comando não encontrado
trace6: commande introuvable
trace6: command not found
trace6: käsku ei ole
trace6: không tìm thấy lệnh
trace6: komanda nerasta
trace6: Kommando nicht gefunden.
trace6: kommandot finns inte
trace6: komut yok
trace6: nie znaleziono polecenia
trace6: níor aimsíodh an t-ordú
trace6: no se encontró la orden
trace6: no s'ha trobat l'ordre
trace6: opdracht niet gevonden
trace6: parancs nem található
trace6: perintah tidak ditemukan
trace6: príkaz nenájdený
trace6: příkaz nenalezen
trace6: команда не найдена
trace6: командата не е открита
trace6: 命令找不到
trace6: コマンドが見つかりません