birdcl6: 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/birdcl6 - from package: bird6

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

Error in other languages:
birdcl6: bevel nie gevind nie
birdcl6: comandă negăsită
birdcl6: comando não encontrado
birdcl6: commande introuvable
birdcl6: command not found
birdcl6: käsku ei ole
birdcl6: không tìm thấy lệnh
birdcl6: komanda nerasta
birdcl6: Kommando nicht gefunden.
birdcl6: kommandot finns inte
birdcl6: komut yok
birdcl6: nie znaleziono polecenia
birdcl6: níor aimsíodh an t-ordú
birdcl6: no se encontró la orden
birdcl6: no s'ha trobat l'ordre
birdcl6: opdracht niet gevonden
birdcl6: parancs nem található
birdcl6: perintah tidak ditemukan
birdcl6: príkaz nenájdený
birdcl6: příkaz nenalezen
birdcl6: команда не найдена
birdcl6: командата не е открита
birdcl6: 命令找不到
birdcl6: コマンドが見つかりません