Here's why:
* Sky Form is a special form exclusive to the Pokémon Black and White games: It's not a form that exists in Platinum, so no code can create it.
* Action Replay codes primarily manipulate game data: They can modify stats, items, and even change Pokémon appearances, but they can't introduce entirely new elements or forms that weren't originally programmed into the game.
What you can do in Pokémon Platinum:
* Obtain Regigigas normally: You can find Regigigas in the Snowpoint Temple after obtaining the Regis (Regirock, Regice, and Registeel).
* Use Action Replay for other modifications: Action Replay can be used to give yourself rare items, increase stats, or even obtain Pokémon not normally available in Platinum.
Remember: Using Action Replay codes can affect your game's save data and potentially lead to unforeseen issues. Use them with caution and always back up your save file before making any changes.