The item Mega Start is not displayed when you have set a Pokémon for which you do not have a Mega Stone in the leftmost spot of your Support Pokémon. To correct this, make sure you place a Pokémon with a Mega Stone in the leftmost support slot.
Mega Start is not available on some stages. If you have a Pokémon with a Mega Stone in the leftmost spot of your Support Pokémon but cannot select Mega Start, the stage you are attempting does not allow Mega Start.
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