In my example, I set direction to Backward, then create a subtask, add the duration of the subtask upward as in the video, and then select a date that cannot be selected
Support Forum
By definition backward scheduled project has a fixed end date and a calculated start date.
It schedules tasks ALAP so a task w/o any constraints will be scheduled to finish at the project end date.
What end date the project has? It looks to me as you are trying to move a task after the project end date which is not possible for automatically scheduled tasks.