With the Pokémon Home service, you can trade Pokémon from your previous games across generations, allowing you fill out your Pokédex on this generation’s Pokémon Sword and Shield, and bring across Pokémon that you’ve been optimizing for your competitive team to the Switch.
The Pokémon Home GTS, intended for players to be able to trade Pokémon from their games amongst the rest of the world, means that picking the Pokémon that you want is easier than ever before.
Most Pokémon found in the world are fine to trade and transfer to the GTS without issue. However, you may be finding that certain Pokémon cannot be placed on a trade on the GTS service. One such exception is the Gen VII mythical Pokémon Meltan and its evolved form, Melmetal. The Steel-type can be found in extremely rare instances in Pokémon Go, but once you have been able to find one, you can transfer it to your Pokémon Home account via the Pokémon Let’s Go games.
When trying to trade it in the GTS though, you will encounter an error message that confirms that the pokémon can not be traded on 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 why they cannot be traded on the GTS is because of their mythical status. These are considered to be extremely rare or one of a kind. It is a challenge to be able to obtain these Pokémon, and allowing them on the GTS service would be an easy way for players to fill out their Pokédex. However, you can trade the Pokémon amongst friends, meaning that you can still obtain them through trading.
Be aware though that despite the fact that the GTS will not facilitate a trade for any of the mythical Pokémon, it will still allow you to request one from other players. However, even when requesting one, you will never receive one in a trade.