In an effort to reduce content misrepresentation, Facebook has removed the ability for Pages to edit how a link appears prior to posting. Effective July 18, 2017, Page editors are no longer able to customize the image, title or description of a link preview.

Facebook has also made changes to other entry points and link post formats:

Page composer. Facebook has introduced a way for publishers to indicate link ownership, allowing them to edit how their own links appear prior to posting.
Ads interface. Facebook is removing the following fields for unpublished link Page posts: Link Headline, Display Link, Description and Picture.
Video link posts. Facebook is removing the ability from all Pages to create organic video link posts.

Read the full article from Facebook for a full explanation of recent changes to Page editor authorization.