address6: 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/address6 - from package: thc-ipv6

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

Error in other languages:
address6: bevel nie gevind nie
address6: comandă negăsită
address6: comando não encontrado
address6: commande introuvable
address6: command not found
address6: käsku ei ole
address6: không tìm thấy lệnh
address6: komanda nerasta
address6: Kommando nicht gefunden.
address6: kommandot finns inte
address6: komut yok
address6: nie znaleziono polecenia
address6: níor aimsíodh an t-ordú
address6: no se encontró la orden
address6: no s'ha trobat l'ordre
address6: opdracht niet gevonden
address6: parancs nem található
address6: perintah tidak ditemukan
address6: príkaz nenájdený
address6: příkaz nenalezen
address6: команда не найдена
address6: командата не е открита
address6: 命令找不到
address6: コマンドが見つかりません