Additional chat-related features and issues within Microsoft Teams for business
Dear Petr Zelenka
Thanks for your quick response!
Based on your further description, Microsoft Teams seems currently has a limitation where you cannot create Planner tasks from messages sent by external users (those outside your organization).
It's great that you identified the root cause! If you frequently need to create tasks from external messages, you might consider using a workaround, such as manually creating the tasks in Planner.
If you have other questions, please feel free to post here.
Thanks for your precious time and your understanding would be highly appreciated. Have a nice day!
Sincerely,
Allen | Community Moderator