bird6: 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/bird6 - from package: bird6

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

On Debian possible default full paths are:

/usr/sbin/bird6 - from package: bird-bgp
/usr/sbin/bird6 - from package: bird

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

Error in other languages:
bird6: bevel nie gevind nie
bird6: comandă negăsită
bird6: comando não encontrado
bird6: commande introuvable
bird6: command not found
bird6: käsku ei ole
bird6: không tìm thấy lệnh
bird6: komanda nerasta
bird6: Kommando nicht gefunden.
bird6: kommandot finns inte
bird6: komut yok
bird6: nie znaleziono polecenia
bird6: níor aimsíodh an t-ordú
bird6: no se encontró la orden
bird6: no s'ha trobat l'ordre
bird6: opdracht niet gevonden
bird6: parancs nem található
bird6: perintah tidak ditemukan
bird6: príkaz nenájdený
bird6: příkaz nenalezen
bird6: команда не найдена
bird6: командата не е открита
bird6: 命令找不到
bird6: コマンドが見つかりません