dhcp6c: 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/dhcp6c - from package: wide-dhcpv6

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

On Debian possible default full paths are:

/usr/sbin/dhcp6c - from package: wide-dhcpv6-client

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

Error in other languages:
dhcp6c: bevel nie gevind nie
dhcp6c: comandă negăsită
dhcp6c: comando não encontrado
dhcp6c: commande introuvable
dhcp6c: command not found
dhcp6c: käsku ei ole
dhcp6c: không tìm thấy lệnh
dhcp6c: komanda nerasta
dhcp6c: Kommando nicht gefunden.
dhcp6c: kommandot finns inte
dhcp6c: komut yok
dhcp6c: nie znaleziono polecenia
dhcp6c: níor aimsíodh an t-ordú
dhcp6c: no se encontró la orden
dhcp6c: no s'ha trobat l'ordre
dhcp6c: opdracht niet gevonden
dhcp6c: parancs nem található
dhcp6c: perintah tidak ditemukan
dhcp6c: príkaz nenájdený
dhcp6c: příkaz nenalezen
dhcp6c: команда не найдена
dhcp6c: командата не е открита
dhcp6c: 命令找不到
dhcp6c: コマンドが見つかりません