Howdy!
We are seeing a spike in reports about attempting to manually log into World of Warcraft will fail with error WOW51900314 if you have an Authenticator attached to your Battle.net account. The current workaround is to utilize the Battle.net Desktop app to launch the game. For security reasons we do not recommend removing your authenticator but that does also resolve the issue.
Our team is currently investigating this issue. We will update this thread once we have more information to share.
Thank you all for taking the time to report this issue to us.