This page explains how to trade from Pokémon SoulSilver to Pokémon: Let's Go, Pikachu!.
Trade Not Possible from Pokémon SoulSilver to Pokémon: Let's Go, Pikachu!
Sorry, but it's not possible to trade from Pokémon SoulSilver to Pokémon: Let's Go, Pikachu!.
Pokémon from older games can't be transferred to Pokémon: Let's Go.