Instructors are allowed to proceed with 2 last cancel runs (goes back one period) or two all cancel runs (goes back to period 0).
The maximum authorized period that you can cancel is 2.
This means that you are not authorize to "cancel all runs" as you are already in period 3.
So when you start on trial training, don't forget that you may use at least one cancel all runs to go back to the beginning of the session.
Instructor > other operations > Cancel last run
Instructor > other operations > Cancel all runs