With the Pokémon Home service, you can trade Pokémon from your previous games spanning multiple generations, allowing you to fill your Pokédex on the Pokémon Sword and Shield, and bring Pokémon you've optimized for your competitive team to the Switch.
The Pokémon HOME GTS, intended for players to be able to trade Pokémon from their games among the rest of the world, means choosing the Pokémon you want is easier than ever.
Most of the Pokémon found in the world can be traded and transferred to the GTS without a problem. However, you may find that some Pokémon cannot be placed on an exchange on the GTS service. One such exception is the mythical Pokémon Gen VII Meltan and its evolved form, Melmetal. The rare type of steel can be found in extremely rare instances in Pokémon GO, but once you find one, you can transfer it to your Pokémon Home account through the Pokémon Let's Go Games.
However, when you try to trade it in the GTS, you will come across an error message that confirms that the pokemon cannot be traded in the GTS, and it would be the same with any creature that is considered mythical. This includes Mew, Celebi, Jirachi, Deoxys, Phione, Manaphy, Darkrai, Shaymin, Arceus, Victini, Keldeo, Meloetta, Genesect, Diancie, Hoopa, Volcanion, Magearna, Marshadow, Zeraora, Meltan and Melmetal.
The reason they cannot be traded on the GTS is because of their mythical status. They are considered extremely rare or unique Pokémon. Being able to get these Pokémon is a challenge, and authorizing them on the GTS service would be an easy way for players to complete their Pokédex. However, you can trade the Pokémon between friends, which means you can still get them by trading.
Be aware, however, that despite the fact that the GTS would not facilitate a trade for one of the Mythical Pokémon, it will still allow you to request one from other players. Be aware that even when you request it, you will never receive a trade-in.
