Currently the tradeable status of rewards in Trainer Challenge are based on the tradeable status of the theme deck's contents used to unlock the reward.
The contents of a deck acquired via Trainer Tokens are non-tradeable, therefore the booster pack reward for completing 12 Trainer Challenge matches with the deck is also non-tradeable. Conversely, the contents of a deck acquired via Gems* or code redemption are tradeable, so the booster pack reward for completing 12 Trainer Challenge matches with the deck is also tradeable.
*Note: As of Oct. 30, 2018, the Pokémon TCG Online phased out the ability to purchase Event Tickets and Gems. For more information please see, "Event Ticket and Gem purchases ending."
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