Hello,
I have checked your site and see a lot of plugins installed on your site that are not related to the theme. Please try to deactivate the third-party plugins and import the demo again.
Note that our dummy content works only on a fresh installation. If you have some content already, it will conflict with your existing posts, and most of the content will not be imported correctly.
If the issue still remains, let me know which demo you want to import so I can check and import the demo on your site.
Best Regards,