dump_dhcp6: 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/dump_dhcp6 - from package: thc-ipv6

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

Error in other languages:
dump_dhcp6: bevel nie gevind nie
dump_dhcp6: comandă negăsită
dump_dhcp6: comando não encontrado
dump_dhcp6: commande introuvable
dump_dhcp6: command not found
dump_dhcp6: käsku ei ole
dump_dhcp6: không tìm thấy lệnh
dump_dhcp6: komanda nerasta
dump_dhcp6: Kommando nicht gefunden.
dump_dhcp6: kommandot finns inte
dump_dhcp6: komut yok
dump_dhcp6: nie znaleziono polecenia
dump_dhcp6: níor aimsíodh an t-ordú
dump_dhcp6: no se encontró la orden
dump_dhcp6: no s'ha trobat l'ordre
dump_dhcp6: opdracht niet gevonden
dump_dhcp6: parancs nem található
dump_dhcp6: perintah tidak ditemukan
dump_dhcp6: príkaz nenájdený
dump_dhcp6: příkaz nenalezen
dump_dhcp6: команда не найдена
dump_dhcp6: командата не е открита
dump_dhcp6: 命令找不到
dump_dhcp6: コマンドが見つかりません