ospf6d: 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/ospf6d - from package: quagga

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

Error in other languages:
ospf6d: bevel nie gevind nie
ospf6d: comandă negăsită
ospf6d: comando não encontrado
ospf6d: commande introuvable
ospf6d: command not found
ospf6d: käsku ei ole
ospf6d: không tìm thấy lệnh
ospf6d: komanda nerasta
ospf6d: Kommando nicht gefunden.
ospf6d: kommandot finns inte
ospf6d: komut yok
ospf6d: nie znaleziono polecenia
ospf6d: níor aimsíodh an t-ordú
ospf6d: no se encontró la orden
ospf6d: no s'ha trobat l'ordre
ospf6d: opdracht niet gevonden
ospf6d: parancs nem található
ospf6d: perintah tidak ditemukan
ospf6d: príkaz nenájdený
ospf6d: příkaz nenalezen
ospf6d: команда не найдена
ospf6d: командата не е открита
ospf6d: 命令找不到
ospf6d: コマンドが見つかりません