Apache: edit to .conf file produces "Invalid command 'Header'"
In order to use Header directive in apache you have to load mod_header module. You can test if module is loaded or not by :-
apache2ctl -M | grep headers_module
find / -name mod_headers.so
If it is loaded you will see something like :-
headers_module (shared)
/usr/lib/apache2/modules/mod_headers.so
If you see no output of find command than load that module directly in your apache conf file. Just append below line :-
LoadModule headers_module modules/mod_headers.so
Note :- mod_header is available as base module in apache. So you don't need to install it explicitly.
Issue following command :-
a2enmod headers
Restart web service
apache2ctl restart