birdc6: 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/birdc6 - from package: bird6

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

On Debian possible default full paths are:

/usr/sbin/birdc6 - from package: bird-bgp
/usr/sbin/birdc6 - from package: bird

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

Error in other languages:
birdc6: bevel nie gevind nie
birdc6: comandă negăsită
birdc6: comando não encontrado
birdc6: commande introuvable
birdc6: command not found
birdc6: käsku ei ole
birdc6: không tìm thấy lệnh
birdc6: komanda nerasta
birdc6: Kommando nicht gefunden.
birdc6: kommandot finns inte
birdc6: komut yok
birdc6: nie znaleziono polecenia
birdc6: níor aimsíodh an t-ordú
birdc6: no se encontró la orden
birdc6: no s'ha trobat l'ordre
birdc6: opdracht niet gevonden
birdc6: parancs nem található
birdc6: perintah tidak ditemukan
birdc6: príkaz nenájdený
birdc6: příkaz nenalezen
birdc6: команда не найдена
birdc6: командата не е открита
birdc6: 命令找不到
birdc6: コマンドが見つかりません