Assigning resources to summary tasks is not recommended. To remove the assignment, double click on it to show the Task Information ...

Assigning resources to summary tasks is not recommended. To remove the assignment, double click on it to show the Task Information dialog box. From the Resources tab in the Task Information dialog box, remove any resources assigned to this summary task.

Note that this will cause the summary task to no longer be shown in the Team Planner view. To associate a team member with a summary task without assigning the task, consider using the Contact field.