Hey! steal_pokemon takes an Array as first parameter (or a Range, please check the documentation : https://psdk.pokemonworkshop.com/yard/Interpreter.html#steal_pokemon-instance_method), so let's say you need to get your Pokémon ID 84 stolen, then you'll have to write :
steal_pokemon([$pokemon_party.pokemon_index(84)], 'lost')
You should make sure your Player does have this Pokémon in their team, or else it'll work weirdly.