[Maruno]

That method is only usable from within events.

You could use this instead:

pbChoosePokemon(1,3,proc {|poke|
   !(isConst?(poke,PBSpecies,:PIKACHU) && poke.form==1)
})
break if pbGet(1)<0

It won't let the player choose a form 1 Pikachu in the first place.