This Pokémon cannot be traded.

Error prevention and placeholders in the Pokémon games:

?????????? | ????? | ! | Abnormal Pokémon | Bad Egg | Enigma Berry (Generation III) | Error! | Error codes (Generations I and II) | Error traps for damaged save data | Error traps to prevent impossible progress | Event data debugging messages | Instant victory effect (Generation II) | Mystery Zone | N/A | Pokégear instruction booklet trap | Rhydon trap | Special (location) | This Game Pak is designed only for use on the Game Boy Color. | This Pokémon cannot be traded.

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The message "This Pokémon cannot be traded" is an error trap and is displayed for when the player attempts to upload a Pokémon with invalid data onto the Global Trading System, or more commonly when the player attempts to upload a hacked or 'abnormal' Pokémon onto the system.

Its purpose has not been fully explained by Nintendo, however it presumably exists to stop players from illegitimate Pokémon onto the Global Trading System. However, not only hacked Pokémon are blocked by this error trap. A similar message will appear when the player attempts to upload a Pokémon with a Classic Ribbon or a Premier Ribbon on to the Global Trading System as "It has a special Ribbon. It can’t be offered for a trade."