You cannot enter Physical Percent Complete into summary tasks.
This behavior is by design. Physical % complete is a calculated value on summary tasks. For the system to calculate it, you must set the earned value method for the project to Physical % Complete and set the subtasks Earned Value Method to Physical % complete and you must assign resources with costs to the subtasks. You must save a baseline, and set your project status date appropriately. The system now calculates Physical % complete.