Your preview session has expired. This is usually due to one of two reasons:
1. It has been over 10 minutes since you looked at a page of your preview since last running the Preview Your Site option. Close this window and in your site administration window click on "Preview/Publish" on the top menu, then "Preview Your Site" on the left-hand menu.
2. If you are seeing this error immediately after running a preview then your browser security settings may be set too high. Make sure that preview.create.net is added to your Trusted Sites. To do this in Internet Explorer, go to the "Tools" menu and click "Internet Options". On the "Security" tab, click the "Trusted Sites" icon, then the "Sites" button below it.