Here's why:
* Trick Room was introduced in Generation V (Pokemon Black and White 2), not in the original Pokemon Black and White.
* TMs in Pokemon White are fixed: The TMs in Pokemon White have a set list of moves, and Trick Room isn't included.
To learn Trick Room in Pokemon White, you'll need to use other methods:
* Breed it: You can breed a Pokemon with Trick Room as an egg move.
* Tutor it: Some Pokemon can learn Trick Room through the Move Tutor. You'll need to find a Move Tutor and see if they offer it.
* Use a cheat device: If you're playing on a home console, you can use a cheat device to give your Pokemon Trick Room. However, using cheats is generally discouraged in Pokemon games.
Remember: If you want to play with Trick Room, you'll need to play Pokemon Black and White 2 or a later generation game.