I remember having a similar problem. It is most likely due to that you are using the patch 43400. Until WoW 4.3.4 is fully downloaded, it should always show that error when a custom patch is installed. And, WoW 4.3.4 is not fully downloaded when you have extracted the torrent. It still needs some extra files, but most of the setup is done.
I could be wrong, but this is how it has worked for me.
I could be wrong, but this is how it has worked for me.