Can't create a Horde character?

It is not created;

What's the error message you get?

Character creation failed...

Maybe you have a client with mods? Try with a clean client.

You could delete/move/rename* cache folder, that might help.
*e.g. cache.bk

You could also try with random hair option/color and see if that helps. Then run to barber and get the haircut you actually want if creation is successful. Just choose the skin color/face you like.

DK stuff EDIT:
Trying to create other DK on standard account would display a different error message that's clear about why it can't be done. Not a "character creation failed".
If you're trying to create a DK on a standard account, you need level 55 character on realm and if I'm not mistaken, no other DK on that realm.

thank you i wiil try~