Apache Invalid command Header
On a new Debian server with Apache, web site give 500 internal server error.
On checking error log, i found
[Thu Jan 04 06:44:42.483932 2018] [core:alert] [pid 27583] [client 112.133.248.19:63020] /home/user/public_html/.htaccess: Invalid command ‘Header’, perhaps misspelled or defined by a module not included in the server configuration
The error is due to Apache headers module not installed on the server.
To fix, run
a2enmod headers
Restart Apache
systemctl restart apache2