I had the same problem, its been fixed now.
What I did: had the prologue working fine (minus a few bugs..), then the game came out, and i downloaded that..
Loaded the game, and i only had Prologue game.. problem.
after lurking on forums, after trying to verify to get nothing I restarted steam. Steam said "Imma download 16gb of game for King Arthur 2, Kay???"
I don't have the download limit to even try that, so instead i paused the game, and tried verifying it. it then was 500mb off, I downloaded that, and its now working..
What I dont understand: In steam before and after, it says its taking 7gb of disk space, which is the size of the prologue campaign.. In the downloads page it says I have the 16gb..
I think that steam isnt liking the Stand Alone expansion (Dead Legions) to be both stand alone AND launching off the same thing..