I tried to trade my Espeon but it says this pokemon can't be traded anymore.

Do you mind giving us a bit more info? Such as which games are involved for example. If one of them is FireRed or LeafGreen it could be because you haven't gotten the Trade Thing in-game to work yet.(The stuff you do on 1st through 3rd Islands after you beat the E4 the first time.)

It's very hard to help if you just say: "Trading Espeon don't work any more." Since we get absolutely no info that could lead to a solution whatsoever.