Import Demo Install Not Working

Hi,
I can’t import demo data because of the error “To protect your content the demo importer only works when WordPress is fresh installed. Please re-install WordPress with the default settings if you want to import the demo data.”.
I’ve already reinstalled WordPress with default settings.
I paid a webmaster to create my website and he does maintenance of the database. I know the credentials but I’m not familiar with it, I only maintain wordpress and modify template settings. So, can’t I use demo pages?
Doesn’t to reset the database means to delete ALL the content of my website? What am I supposed to do? Erase the whole 2-years-old-website?

Is it possible to reset the database, import the demo data and then reconstruct the old database whit all of its content?

I know this theme is fully customizable but I’ve bought it because I liked the demo structure. I’ve already used other templates, but I’ve never had to reset the database to use it.
Database reset isn’t marked as a prerequisite before buying the template.

Thank you.