WP Error message about fixing my theme

After installing the plugin today, I had this error at the top of your plugin page.

Error : Please fix your theme to make plugins work correctly: Go to the Theme Editor and insert <?php wp_head(); ?> just before the </head> line of your theme’s header.php file.

I am using a Child Theme and the only place I can find the header.php file is in the main theme.

However, there is no /head tag in that short file.

Therefore how can I get this to work and remove the error.

The page I need help with: [log in to see the link]