Most things in xcraft are either own section which is accessible by a shortcut button button on the left or on the top - or they are info panels that can be closed again to return where you were.
Battle simulator is neither. It is not accessible from the shortcuts and it cannot be closed.
The battle simulator is only accessible from espionage info panel and fleet section and it would make sense for it to be an info panel so you can run the simulation, close the simulator, and perform the attack.
Instead you have to select some random other thing, look up where you were before running the simulator, and then perform the attack.