You cannot query on the Activity History object (see http://community.skuid.com/t/can_i_create_a_list_of_activity_records)
You should build your activity reporting on the task object itself. This won’t necessarily let you see churn on one particular task, but if you do you filtering on last activity date, or due date you can see at least one count for each task.
I assume you are following along with Anna’s tutorial at http://help.skuidify.com/s/tutorials/m/models-conditions-filters/l/102652-aggregate-models-make-dyna…