Here's why:
* SkullGreymon is a Digimon from the "Digimon Adventure" series and its subsequent adaptations. It's not a part of the Digimon World Dawn/Dusk storyline.
* Digimon World Dawn/Dusk have their own unique Digimon roster. While some Digimon are shared between series, there are many exclusive to each game. SkullGreymon is simply not one of them.
To evolve your Digimon into a powerful form in Digimon World Dawn, you'll need to focus on the Digimon available in the game. You can find a list of available Digimon and their evolution paths online or in dedicated Digimon World Dawn wikis.
If you're looking to experience SkullGreymon, you'll need to play a different Digimon game that includes it, such as Digimon World 3 or Digimon Adventure.