Every Realm shows Incompatible under the population Header during login. The list shows that I have 12 toons on Windrunner and I cannot select it. Are the servers full up or is there a different problem on my end? --update-- ended up uninstalling and reinstalling battle.net app.
In future, there’s a much quicker solution. This indicator means that a minor patch has been released, but you haven’t yet applied it to your client. You can force this to be detected from within the Blizzard App by selecting “Options” on the WoW page and then “Check for Updates”.