license for Premium Planner to complete the tasks

An E3 user does not need a premium Planner license merely to complete an assigned task in a premium plan, provided their subscription includes Planner and its service is enabled. Microsoft’s Planner FAQ explicitly allows Planner in Microsoft 365 users to view and edit tasks in premium plans created by appropriately licensed users. Those editing capabilities include assigning tasks, changing dates, and adding attachments. The premium license is needed for advanced capabilities, not simply because the plan itself is premium. First verify the assignee’s Planner entitlement and access to the relevant plan, then test opening the assigned task and marking it complete. If access fails, capture the exact error and check membership or sharing before purchasing another license. Features such as Timeline, People, Goals, and other advanced planning tools have separate licensing requirements. Therefore, license each user for the capabilities they actually need.