In Red Sparrow Dominika Egorova attacked a fellow cadet, who tried to rape her. Later, Matron calls the fellow cadet and her. Matron says to her:
For the deception to be complete, you must hold nothing back. Nothing. Understand? Give him what he wants.
What deception is Matron referring to?
You seem to be focusing on the meaning of the words spoken and ignoring the context of the scene surrounding them.
The scene in question takes place in the middle of a classroom. The school they're in is an SVR (Russian Intelligence) school for training female assassins, and the specific class they are attending is about seduction, and how to use it to manipulate targets of interest.
There is no deception between Dominika and the cadet she's paired with (they both know what's going on in the moment), but Matron is teaching a class about how to seduce people, which seria involve deception, out in the real world.
The deception she refers to is the act of deceiving the hypothetical victim they will eventually be using these skills on in the future.