So I caught the shiny Celebi from the research in Pokémon Go. I used to Pokémon transporter to being it to HOME on my phone. When I go to my switch and try to bring the Celebi to Shield, it says “The selected Pokémon brought over from Pokémon GO has not been obtained in this save file. The Pokémon cannot be withdrawn.” What does that mean? Why can’t I bring the Celebi to Shield? How do I fix this?
In order to bring certain Pokémon caught in Pokémon GO and transferred to Pokémon HOME into the Pokémon Sword or Pokémon Shield game, you must either have registered that Pokémon’s species to your Pokédex or have obtained a Pokémon of that species at any point in that game.
It seems you need to have previously owned a Celebi to transfer this specific one from Pokemon GO into Sword/Shield.
Okay I figured it out: Mew has to be in your party/boxes for you to transfer it out of Home. Mew has no Dex entry so this is the only way to keep it "registered," for you to be able to take it out.
This also applies to Celebi.
Source 1 Source 2 (Recommend reading this one)