On my twice account i bought a terraria on xbox 360 and i dont have it on xbox one why?...

In that case you might think of it like this:

You bought a movie on a VHS Tape (Terraria on the Xbox 360).

But you are unable to play the VHS Tape on your BluRay player (Xbox One).

Even though it's the same movie, the format is different.

Some publishers offered discounts or ways to get the upgraded Xbox One version of their games, especially during the launch of the Xbox One, but those offers were specific for the game and the publisher.

Unfortunately, Terraria is not such a game, and you would have to buy it again to be able to play it on the Xbox One.

A growing number of Xbox 360 titles is being made backwards compatible, allowing the Xbox One to play the Xbox 360 version of the game. So if the Xbox 360 version of Terraria becomes backwards compatible, you can play your Xbox 360 version for free on the
Xbox One.

But the Xbox One version of Terraria is a separate purchase.

You might try to contact the publisher, if they have a special upgrade offer for you.

There are two different versions of the same game as decided by the game developer; one for xbox 360 and the other for xbox one. You need the xbox one version of the game to play it for xbox one.

but it dont make sens for me because terraria on xbox 360 and xbox one is the same game ! its not changing???

To play Terraria for xbox one you need to purchase the xbox one version of the game.

I dont have my xbox 360 anymore, im xbox one so i dont understanf why i lose all my achievements on 360