Sub menus don´t work.

As you can see, when you click on the sub menus, it appears an error, and I don´t know how to fix it. The template is Zeever and I´m using Gutenverse. Also, another question is, when i create a page, it doesn´t let me change the background photo, because it doesn´t even appear in the edit mode. When you create a page it appears the text saying post title (which i can´t delete), and if i add a background, it appears twice. Please help

The page I need help with: [log in to see the link]