The explanation wasn't too clear, but there are several reasons it won't Mega Evolve.
Firstly, it needs to be holding it Mega Stone - in this case the Charzardite Y
Secondly, you need to have progressed far enough in the story to have gotten the Mega Ring. You get this on the Tower of Mastery just after you have beaten he Shalour City Gym.
Thirdly, have you already Mega Evolved another Pokemon in that battle? If so, then you can't Mega Evolve anyone else until the battle ends.
My guess is that you haven't done one of these three things (or have done in the case of the 3rd one)
I hope this helped.