Hi there,
Sites are really designed to be installed on a clean installation, not an existing site. As the results may not be as expected. Therefore advisable to back up an existing build if you want to try to import a site on to it.
This guide shows you how to clean an existing Site ready for a new install, please note that this would remove all content and therefore you may just want to manually remove the Additional CSS from the Customiser, remove the plugins installed by the Site, remove widgets and menus created by the site.
https://docs.generatepress.com/article/removing-imported-site/
But please make sure you have backups before doing so.