dhcp6ctl: 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/sbin/dhcp6ctl - from package: wide-dhcpv6

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

On Debian possible default full paths are:

/usr/sbin/dhcp6ctl - from package: wide-dhcpv6-client

You may install the required package with command apt-get install {package}

Error in other languages:
dhcp6ctl: bevel nie gevind nie
dhcp6ctl: comandă negăsită
dhcp6ctl: comando não encontrado
dhcp6ctl: commande introuvable
dhcp6ctl: command not found
dhcp6ctl: käsku ei ole
dhcp6ctl: không tìm thấy lệnh
dhcp6ctl: komanda nerasta
dhcp6ctl: Kommando nicht gefunden.
dhcp6ctl: kommandot finns inte
dhcp6ctl: komut yok
dhcp6ctl: nie znaleziono polecenia
dhcp6ctl: níor aimsíodh an t-ordú
dhcp6ctl: no se encontró la orden
dhcp6ctl: no s'ha trobat l'ordre
dhcp6ctl: opdracht niet gevonden
dhcp6ctl: parancs nem található
dhcp6ctl: perintah tidak ditemukan
dhcp6ctl: príkaz nenájdený
dhcp6ctl: příkaz nenalezen
dhcp6ctl: команда не найдена
dhcp6ctl: командата не е открита
dhcp6ctl: 命令找不到
dhcp6ctl: コマンドが見つかりません