The TASKS by Planner Add-In App for Microsoft Teams are not the same as a Plan in the Microsoft Planner App. Plans created in Planner do not import to TASKS by Planner, you must import the plan to a Team in Microsoft Teams.
To Add a Plan created in Planner to a Microsoft Teams Team, follow the instructions on this page:
https://techcommunity.microsoft.com/t5/planner-blog/bringing-a-plan-into-microsoft-teams/ba-p/57463