Read on below to learn about why UX / Mega-Evolved Pokémon might not show up in the Edit Decks screen in Pokémon Duel:
UX figures include Mega Evolutions of other figures. These figures can’t be added to your deck directly. Instead, you will need to link it as an Evolution. For instance, if you have [UX] Mega Charizard X, you will need [EX] Charizard so that you can set up an Evolution for it to evolve. Once you have set up the Evolution, you can then add Charizard to your deck and then be able to use it and its Mega Evolution in battle.
You will also need the plate that represents the Mega Stone to Mega Evolve in battle. For the example above, you would need the Charizardite X plate to Mega Evolve Charizard in a duel.
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Source: Official Pokémon support