I got the following error for one of the Magento e-commerce website:
Invalid command ‘Header’, perhaps misspelled or defined by a module not included in the server configuration
After googling it was found that the related headers module was not enabled in Apache.
I was able to fix the issue simply by following two commands:
1. Enabling headers Module
ln -sf /etc/apache2/mods-available/headers.load /etc/apache2/mods-enabled/headers.load
2. Restarting Apache
Note: Above commands were run from Ubuntu 10.04.2 server
Hope this’s helpful.