implementation6d: 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/implementation6d - from package: thc-ipv6

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

Error in other languages:
implementation6d: bevel nie gevind nie
implementation6d: comandă negăsită
implementation6d: comando não encontrado
implementation6d: commande introuvable
implementation6d: command not found
implementation6d: käsku ei ole
implementation6d: không tìm thấy lệnh
implementation6d: komanda nerasta
implementation6d: Kommando nicht gefunden.
implementation6d: kommandot finns inte
implementation6d: komut yok
implementation6d: nie znaleziono polecenia
implementation6d: níor aimsíodh an t-ordú
implementation6d: no se encontró la orden
implementation6d: no s'ha trobat l'ordre
implementation6d: opdracht niet gevonden
implementation6d: parancs nem található
implementation6d: perintah tidak ditemukan
implementation6d: príkaz nenájdený
implementation6d: příkaz nenalezen
implementation6d: команда не найдена
implementation6d: командата не е открита
implementation6d: 命令找不到
implementation6d: コマンドが見つかりません