So I have a lv. 50 Charizard in my Pokemon Shield Box, and I cannot move it into Lets go Eevee. If more Info is required, please ask.
Only Pokemon from Let's Go Pikachu and Let's Go Eevee can be moved into the mentioned Let's Go games. If a Pokemon comes from or is one day placed into a game other than those two, then they cannot be transferred to LGPE. Since your Charizard is from Shield, it cannot be transferred to Let's Go Eevee. You may refer to this diagram from the official website for a simplified summary of compatibility.
Sources: https://home.pokemon.com/en-us/move/ https://bulbapedia.bulbagarden.net/wiki/Pokémon_HOME#cite_note-lgpe-twoway-4