If you get an error when creating the Death Knight, and you have met the requirements of having a level 55 character on the server in which you're creating the Death Knight, just make a new level 1 character first.
After that character is created, you can then create your Death Knight without any more problems.
I have to wonder how the bug came to be, and why they weren't able to fix it. We probably won't know either of those answers. But at least we have a quick and dirty fix.