i leveled up mewtwo with an rare candy to learn me first but he didnt learn it what happened.
It learns Me First in Gen IV games at LV 79, and learns it at LV 71 in the Gen V games. Maybe you mixed the games up, or your game is glitched.
Well that depends. Before Gen IV (4) that move did not exist.So that sanswers it for Emerald and games before then. But in Gen IV it learns it at 79. But if you did it in Black/White/B2/W2 then your games is messed up.