Just want to clarify that this discussion forum is accessible by all players with an active WoW subscription.
I’m not a qualified tech supporter by any stretch, but I had that issue one time following an inelegant shutdown due to a sudden power outage. The solution for me in that case was to go to my /World of Warcraft/ folder and delete the entire /Data/ subfolder. I think there was a corrupted file in there. (But I’m not certain about it!)
From that point, the Battle.net App regained the ability to patch the game up.