Hi @Dalia Khan,
This problem can occur if one of the following conditions is true:
- The file is in the Checked out state.
- The file is in the Draft state, and the library’s Draft item security options don’t include Any user who can read items.
To fix this problem, the file should be checked in or moved from the draft state. If the issue persists, it may be due to permissions not being granted to the user's account to access the sites, library, or list in which the user wants to use @mention functionality.
Reference: @Mentions in SharePoint files don’t send email notifications to participants.
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