'Cannot share a link to this site' error in the Share a link blueprint in Confluence Server
Platform notice: Server and Data Center only. This article only applies to Atlassian products on the Server and Data Center platforms.
Support for Server* products ended on February 15th 2024. If you are running a Server product, you can visit the Atlassian Server end of support announcement to review your migration options.
*Except Fisheye and Crucible
Problem
When using the Share a link blueprint, the user sees the following error:
You cannot share a link to this site. Ask your admin to add it to the allowlist.
Cause
A URL or domain must be added to the allowlist before it can be shared using the Share a link blueprint. This is to prevent people from accidentally or maliciously sharing links that may pose a security risk to your site, as the link contents is previewed by the blueprint.
This approach is also used in other parts of Confluence where content from any website can be displayed within Confluence, such as gadgets, the RSS feed macro, and the HTML include macro.
Resolution
Go to Administration menu 
, then General Configuration > Allowlist and add the URL or domain to Confluence's allowlist. You'll need System Administrator global permissions to do this.
See Configuring the Allowlist for more information.