Hello,
Please try to deactivate the third-party plugins not related to the theme and import the demo again.
If the issue remains then They may be caused by low PHP configuration limits. Try to Increase the “PHP Max Input Vars” parameter to 10000 or higher in your PHP configuration and import the demo again.
If the issue still remains, share the wp-login 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,