PokeFarm is a breeding simulator, and Pichu evolves by leveling up to level 15 with high happiness. While you can breed Pichu in PokeFarm, the game doesn't have a mechanic for raising happiness or leveling Pokemon.
If you want to evolve Pichu, you'll need to play a game like Pokemon GO, Pokemon Sword/Shield, or Pokemon Brilliant Diamond/Shining Pearl, where you can level it up and increase its happiness.