Is Blastoise holding the Blastoisinite?
Make sure Control Switch for Mega Evolution is turned ON, I think it's 32 or 34, around there...
Sooo switch 34 is "no mega evolution" and 32 is fateful encounters :/
I don't think you need to activate a switch. At least, I'm able to mega evolve in a fresh save file as long as I have a mega ring and the Pokémon is holding its mega stone.
What version of Essentials are you using?
I'm using v18.1 but when I enter a fight I don't see the mega "button"
Are you using any custom scripts that might affect the battle? Like Dynamax or the Gen 8 project?
Yeah I had the Gen 8 project I guess, and recently like 20 mins ago added the Dynamax