Your theme won’t look exactly like the demo version right away. Demo sites are full of text and professional photography to showcase everything the theme is capable of. You’ll need to create pages and posts, and start adding your own content in order to see the site start to take shape. Also, you can import the demo content and then replace the text and images with your own – no coding required!
Another reason would be that Reading settings are wrong. This means your theme either does or doesn’t require a home page to be set, and you have your site set to the opposite. So go to Settings > Reading and check the options under “Your homepage displays”:
If your theme doesn’t require any page template for the homepage layout, make sure this option is set to “Your latest posts”. If your theme does use a homepage template, have your site set to use a static homepage.