dhcp6s: 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/dhcp6s - from package: wide-dhcpv6

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

On Debian possible default full paths are:

/usr/sbin/dhcp6s - from package: wide-dhcpv6-server

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

Error in other languages:
dhcp6s: bevel nie gevind nie
dhcp6s: comandă negăsită
dhcp6s: comando não encontrado
dhcp6s: commande introuvable
dhcp6s: command not found
dhcp6s: käsku ei ole
dhcp6s: không tìm thấy lệnh
dhcp6s: komanda nerasta
dhcp6s: Kommando nicht gefunden.
dhcp6s: kommandot finns inte
dhcp6s: komut yok
dhcp6s: nie znaleziono polecenia
dhcp6s: níor aimsíodh an t-ordú
dhcp6s: no se encontró la orden
dhcp6s: no s'ha trobat l'ordre
dhcp6s: opdracht niet gevonden
dhcp6s: parancs nem található
dhcp6s: perintah tidak ditemukan
dhcp6s: príkaz nenájdený
dhcp6s: příkaz nenalezen
dhcp6s: команда не найдена
dhcp6s: командата не е открита
dhcp6s: 命令找不到
dhcp6s: コマンドが見つかりません