Here's why:
* Beat Up is a move that is only available to certain Pokemon. While Charmander can learn a variety of moves, Beat Up is not one of them.
* Move availability is determined by the Pokemon's species. Each Pokemon has a specific set of moves it can learn either by leveling up, through TMs, or by breeding.
You can check the move list for Charmander in Pokemon Pearl to see which moves it can learn.
To find a Pokemon that can learn Beat Up in Pokemon Pearl, you could try:
* Looking up the move's availability on a Pokemon database: Websites like Serebii.net or Bulbapedia list all the Pokemon that can learn specific moves.
* Finding a Pokemon that learns it through level-up: Some Pokemon learn Beat Up naturally as they level up.
* Trading for a Pokemon that knows Beat Up: You can trade with other players to get a Pokemon that already knows the move.
Good luck catching the Pokemon you need!