When I went to the jade tower and challenged Taen for the first time I went up the stairs and saw the item, and when I picked it up I instead received a Charzardite Y, at first I was like Hell Yeah! But I realised it was supposed to be your starter’s mega, So I was wondering if I can get it later on in the game, if someone could help me out that would be much appreciated 
Is your starter Delta Charmander? If so it could be because you turned on Purity mode after you got the starter.
Yeah I chose charmander, if that is the case Can I get it any other way?
You can ask someone for a trade?
You can buy it in the battle frontier, though that’s postgame.
Well it looks like I messed up then, thinking that Purity mode was to turn on the new megas, I’m so cut rn but not much I can do about it
If you haven’t done too much since the you can look through backup saves to find one ebfore you pick up the Mega Stone.