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] /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

Restart Apache