You have modified the configuration files in Apache, you try to restart the Apache server in order to apply these changes, however you get the following error : "Invalid command header". That means that the headers module of Apache has not been enabled.
To apply the headers module :
sudo a2enmod headers
Then you restart Apache :
sudo systemctl restart apache2
Finally it should work.
Note: Commissions may be earned from the links above.
This page contains references to products from one or more of our advertisers. We may receive compensation when you click on links to those products. For an explanation of our advertising policy, please visit this page.