header.py: 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/lib64/xulrunner-devel-17.0.10/sdk/bin/header.py - from package: xulrunner-devel
/usr/lib/xulrunner-devel-17.0.10/sdk/bin/header.py - from package: xulrunner-devel

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

On Debian possible default full paths are:

/usr/lib/icedove-devel/sdk/bin/header.py - from package: icedove-dev
/usr/lib/iceweasel-devel-38/sdk/bin/header.py - from package: iceweasel-dev

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

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