How to fix Invalid command ‘RequestHeader’ in the server configuration

Invalid command ‘RequestHeader’ error occurs in the Linux server due to missing apache modules in our Linux server.

Invalid command 'RequestHeader', perhaps misspelled or defined by a 
module not included in the server configuration

To fix this issue, just enable the “headers” module. If you are using Ubuntu just enable the module by running the following command:

sudo a2enmod headers

After that. Restart apache

service apache2 restart