Error

Hi,
When I login at the backend of my site, below is the error msg I see on top:

“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.”

But when I try to make changes as said above, I don’t see the </head> to where I have to make changes or anything.

My Site: knowledgemasti.com

I am not sure if one of the plugin that I downloaded causing this or why this message pop up and if this going to harm my site in any way.

Please Help!!

Thank You
Nik