I cannot tag Users in Planner and get an Error

20°26"07.080

-21°42"48.729

Hi Victoria, 

I’m sorry you’re going through this issue, and I appreciate you sharing the details of what happened. 

As a forum moderator, I genuinely wish I could directly access your account or review the backend systems to diagnose and resolve this issue for you. However, our role here is limited to providing general guidance by offering clear next steps, sharing applicable resources, and directing you to the appropriate support channels so you can work with the right team to investigate further. 

Based on your description, it looks like the issue is specifically related to using @mentions / tagging users in Planner task comments. Since you’re able to post a comment successfully when no one is tagged, your general ability to comment on the task appears to be working. The failure seems to occur only when Planner tries to process the tagged user and send the related notification. 

To help narrow this down, could you please try the following checks? 

1. Confirm the tagged user has access to the plan/task 
   Please make sure the person you are tagging is a member of the plan or has access to the task. 
   • Open the affected Planner plan and check the plan/group member list to make sure the user you are mentioning is included.  
   • If the plan is connected to a Microsoft Teams team, also check the Team membership under Manage team > Members. Users can only be mentioned in Planner task chat if they have access to the plan/task. 
2. Test tagging another user 
   • Try mentioning a different user in the same task. This will help confirm whether the issue happens with all users or only a specific person. 
3. Test in different locations 
   Please check whether the issue happens in: 
   • Planner on the web 
   • Planner inside Microsoft Teams 
   • A different browser or an InPrivate/Incognito window 
   • 
4. Clear browser cache or try another browser 
   If the issue is browser/session-related, this may help resolve it. 

5. Try a shorter comment with only one mention 
   For example: 
   “@UserName please check this task.” 
   This helps rule out formatting or message content issues. 

For your references: 

• Comment on tasks in Microsoft Planner | Microsoft Support
• Add people to your plans | Microsoft Support

If the issue persists even after these steps, it may stem from a deeper system-level cause. In this case, I recommend contacting your organization's IT administrator, who can submit a support request to Microsoft Support through the Microsoft 365 Admin Center. They’ll be able to access system-level information to investigate the issue thoroughly and provide assistance. At the very least, they can offer the most effective workaround to ensure your experience remains smooth and secure.      

Please also share the affected user’s email address, the user being tagged, whether the issue occurs with all tasks or only one, whether other users in the same plan can tag successfully, the browser name and version, and a screenshot of the error if possible. 

• In case you do not know who is your IT admin, kindly refer to this article: How do I find my Microsoft 365 admin? - Microsoft Support  

I hope this information is helpful. Should you have any further questions or need additional assistance, please feel free to share them in the comment below. I'm very happy to help.  

Thank you again for your patience and understanding. 

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