Hi RussoMaria-2416,
Only task created in the same Team can be used by channels. You can choose “Use an existing list from this team”.
For more details, you can refer to this link: https://support.office.com/en-us/article/use-planner-in-microsoft-teams-62798a9f-e8f7-4722-a700-27dd28a06ee0.
If it cannot meet your requirements, we recommend you that please give a feedback to https://microsoftteams.uservoice.com/forums/555103-public/suggestions/40486495-tasks-assigned-in-ms-teams-planner. Microsoft will always focus on customer’s feedback and experience. Some new features would be added to the services based on customers' feedback in the future, and your good ideas will be very helpful for them to improve the service.
Your time, understanding and cooperation will be highly appreciated.