Internal Server Error - htaccess

the Header directive is in the mod_headers apache module. You need to make sure that module is loaded into the apache server.

Steps to enable HEADERS modules

$ cd /etc/apache2/mods-available
$ sudo a2enmod headers
$ /etc/init.d/apache2 restart

On your command line:

Install mod_headers

sudo a2enmod headers

and then restart apache

service apache2 restart