If your Battle.net client won't give you a "Start" button, try this:

[VitaKaninen-1422]

See this post here for more info:

[PC] Unable to Log In with Digital Deluxe or Ultimate Edition Early Access? Info Here!

Hey all! We’ve gotten some reports that the Battle.net Application isn’t updating correctly with Early Access status. If this applies to you, try the following. Ensure that you have purchased the Digital Deluxe or Ultimate Edition of the game by checking your orders and games page. Note that the standard edition does not get early access Restart your PC Change the battle.net application’s region. To do this, click your BattleTag in the top right, choose “Log Out” then use the settings cogwhe…