Max Pokemon lvl is 39. Why not 40?

Normal Because when your reach lvl38, you can only power up once per level your pokemons instead of 2 times before.

So that It's 39,5 when your are lvl39 and 40 when your are lvl 40

Pokemon's max level is MIN{40, Trainer level + 1.5}.

Since you're level 38, you can't train a pokemon to level 40. No different than a level 12 can't train a pokemon to level 15.

Get to level 39, then you'll be able to.

How you get to 38 so fast? Its only been like 4 months the game has been out?? so lucky.

The game code caps the pokemon level at 40, but there is a client side bug that causes the power up button to be removed from the pokemon info screen early at level 39 that Niantic hasn't bothered to fix yet.

Apparently if you are logged into two devices at the same time (not sure whether or how this violates ToS), you can look at the same pokemon on each device, press the power up button on one to get to lv39, then press the power up button on the other to get to lv39.5. Presumably if you had three devices, then you could force it up to lv40, but I haven't heard any actual reports of people doing that.