What template is that? Did you create it or buy it from a third party?

These FAQs might help explain the template system to you. However, if you have a third-party template, they might have deactivated some functionality.

https://www.zen-cart.com/tutorials/i...hp?article=230
https://www.zen-cart.com/tutorials/index.php?article=36
https://www.zen-cart.com/tutorials/i...hp?article=231
https://www.zen-cart.com/tutorials/i...hp?article=232

Usually, that "empty" space is a defined file. Found in the admin panel under Tools --> Define Pages Editor. But again, if you're using a third party template.... YMMV.

HTH,