If you are seeing the error message when you are trying to open a url. Sorry, you are not allowed to preview drafts. The message is very clear you are either not loged in or you are trying to open a broken or expired preview link. While the error is very clear and tells what’s the problem is. But sometime it can be confusing if you are not paying proper attention to what’s going on.
Please note the previews are made only for authors or admins or loged in users so they can see the changes before make them live or publish them. So if you are not loged in then you would not able to preview the drafts at all.
When you are not loged in then you should use a proper permalink instead of preview link.
Steps to clear error you are not allowed to preview drafts.
First of all you should know only you can preview only when you are loged in. If you are not loged in then you can not see the preview of your draft.
If by mistake you have copied the URL of preview and linked it somewhere in site then edit that link and remove preview query strings from URL.
You can just remove this from your URL and the problem would go away.
?preview_id=886034&preview_nonce=7f61dddba1a48&preview=true&_thumbnail_id=6040
In the example above you may see the different in numbers but ignore that and just remove everything after ? mark and you can then see the url normally.