If you get the error message…
Error: Please fix your theme to make plugins work correctly. Go to the Theme Editor and insert the following string: Please enter it just before the following line of your header.php file: Go to your theme editor: click here.
…then please follow the steps as outlined in the error message. If you need help, please have a look at this article.
As outlined in the above-mentioned article, every well-developed theme should already have the
wp_head functions as they are needed by many plugins (not just ours). It is a standard in WordPress. Also, see this article.