implementation6: 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/implementation6 - from package: thc-ipv6

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

Error in other languages:
implementation6: bevel nie gevind nie
implementation6: comandă negăsită
implementation6: comando não encontrado
implementation6: commande introuvable
implementation6: command not found
implementation6: käsku ei ole
implementation6: không tìm thấy lệnh
implementation6: komanda nerasta
implementation6: Kommando nicht gefunden.
implementation6: kommandot finns inte
implementation6: komut yok
implementation6: nie znaleziono polecenia
implementation6: níor aimsíodh an t-ordú
implementation6: no se encontró la orden
implementation6: no s'ha trobat l'ordre
implementation6: opdracht niet gevonden
implementation6: parancs nem található
implementation6: perintah tidak ditemukan
implementation6: príkaz nenájdený
implementation6: příkaz nenalezen
implementation6: команда не найдена
implementation6: командата не е открита
implementation6: 命令找不到
implementation6: コマンドが見つかりません