It looks like it points to a navigation layout which doesn;t existt in your project. I would change this in properties:
Than drag down your content to the content container of the selected navigation layout and it shoudl work fine.
This is the page after I import it. What should I do next?
When I enter the URL, I get the corresponding JSON code.
Click in the white area outside of the container. You should see the properties somewhere on your right hand. Maybe it opens the tab toolbox or something like that but than select the properties tab and you're fine. So click Layout and select a navigationlayout from for example "UI Resources" or any layout you want to use. In my example i type in popup and i see several options popping up which I can select.
May I suggest another thing, it would be good for these kind of things to also try the learning path Mendix offers, I'm not sure how far you are with your training but the "Becaome a rapid developer" might be great to follow to further accelerate your knowledge.
EDITED: Added a printscreen of the Select Layout option.