You can’t get there from here (site_id mismatch error). This happens when an external blog (a self-hosted blog using the Jetpack plugin) is pointed to WordPress.com by DNS. If you are trying to migrate away from WordPress.com, please update your DNS to point the domain to your own server. Otherwise please contact WordPress.com support and explain what you were trying to do, what steps you took, what you expected to happen, and what actually happened.