I hope you are well today and thank you for your question.
Are you trying to import the Basic demo content as shown in the attached screenshot?
If not then could you please share the screenshot of the demo content that you are trying to import?
Also, would you mind if I log in to your site and do quick troubleshooting? If this is ok then could you please temporarily create an admin user account and share the account login details privately by adding them in the box having text “Enter your private content here (only you and forum moderators will be able to see it)”?