Hello,
Please try to deactivate the third party plugins and import the demo again.
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
Otherwise, if the issue still exists then please share the WP admin login details of your site so I will check and import the demo on your site. Also tell me which demo you want to import.
Best Regards.