I have a previous post which demonstrates how to add HTTP Authentication by .htaccess.
.htaccess – Setting Password For Your Web Folder

But if it does not work and shown the following error in the Apache error log

• /home/ykyuen/public_html/.htaccess: Invalid command ‘AuthUserFile’, perhaps misspelled or defined by a module not included in the server configuration

 

You can fix it by turn on the authn_file module. For Debian Linux like Ubuntu,

• a2enmod authn_file
• /etc/init.d/apache2 restart

 

For Fedora, add the following line to /etc/httpd/conf/httpd.conf

• LoadModule authn_file_module modules/mod_authn_file.so

Then restart the Apache

• /etc/init.d/httpd restart

 

Done =)

Reference: readthefuckingmanual.net – Invalid command ‘AuthUserFile’, perhaps misspelled or defined by a module not included in the server configuration