Hi @Ken Lin ,
Thank you for posting in this community.
Unfortunately,This feature only works for people who are in the same Microsoft 365 group as you. If you want to assign tasks to external users, you need to add them to your group first. You can do this by following these steps.
Please refer to this article:https://insider.microsoft365.com/de-de/blog/mentions-for-comments-in-word-excel-and-powerpoint-available-to-insiders-fast-slow
Microsoft eager to hear your feedback! You can leave comments on this post or submit feedback in Word/Excel/PowerPoint, Many features of current products are designed and upgraded based on customers’ feedback:
- On Windows go to File > Feedback
- On Mac go to Help > Feedback
- On iOS/Android go to Settings > Send Feedback
- For Office Online go to Help Improve Office
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