Hello,
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 remains then They may be caused by low PHP configuration limits. In this case, you need to increase some values.
memory_limit 128M
post_max_size 64M
upload_max_filesize 64M
max_execution_time 180
max_input_vars 10000
If the issue remains, share the wp logins details of your site and tell us which demo you want to import so i will check and import the demo on your site.
Best Regards.