Once I transferred one site from one server to another Ubuntu server and after installing Apache2 I noticed the following error in the logs:
Invalid command ‘Header’, perhaps misspelled or defined by a module not included in the server configuration
The error occurred due to the fact that the headers module was not activated, check if it is loaded (“headers_module (shared)” should be displayed):
apachectl -M | grep head
To activate it, you need to run the command:
sudo a2enmod headers
And restart apache2:
sudo service apache2 restart
After that, the error disappeared.
See my other articles on Apache2