To stop the spread of misinformation and false news on their platform, Facebook has decided to remove the ability to edit previews attached to link posts, both natively and via third-party publishing tools like Agorapulse. You can read their official announcement here

We've been authorized to keep offering this feature for the last few months: unfortunately, this transition is now over for all third-party publishing tools starting on December 18, 2017.

1. Publish links from other websites

​It is no longer possible to edit the thumbnail, title or description of a link preview in a Facebook post.
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2. Publish links from your own domain

You can make sure your own website is set up to generate optimal link previews: see Facebook documentation here

Users who have verified their domain(s) in Facebook Business Manager will be able to continue editing link metadata for links that point to verified domains. If you do not verify your domain on Facebook, we will publish default metadata, even for your own website.

To verify your domain, please refer to Facebook's documentation.

If you are publishing the same post on multiple Facebook Pages, you will be able to customize the link details only for the Page that owns the given domain. Changes to the link details will not apply to other Facebook Pages that do not own the domain.