Theme and Ultimate Social Media and Share Icons Conflict

Hi,

I am receiving the following error:

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:
Theme: CustomizrVersion: 3.4.8 (with a child theme – though I get the error with and without the child theme)
Plugin: Ultimate Social Media and Share Icons Version 1.2.5
WordPress: 4.3

The plug-in Ultimate Social Media replied to contact Customizr. I do not have </head> line in my header.php file.

Help!
Thank you
Diana