Error : Please fix your theme to make plugins work Correctly

HI Guys!!! I'm desperate .... this seems like a nightmare ..
I have a terrible problem, I can not perform any upgrade of any plugin, or update wordpress , because when I try wordpress administrator turns in a withe screen, .... then I switch from FTP plugins and then re- activate them one by one, so that I can go back to see the manager wordpres

On top of wordpress is a message that says I have an 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 . "

But I do not know where else I can make this change , because when I try to go to my theme editor returns to the blank page.

Pleeeaaase help me!!! :

Hi,

Try to do it via FTP client. Do you have any modifications in theme files? If no - please upload the latest version of the Simplicity theme into your wp-content/themes directory. Make a full backup first!

Hello thank you very much! everything seems to have worked very well with the solution that you gave me .

I just have one more question , woocomerce plugin , I do not use it on my site ... it is necessary to have active or could eliminate ? ... This could affect its functioning ?

Sure if you are not using the WooCommerce, you can deactivate and remove the plugin without the influence on theme's functionalities.