"Start Game" option grayed out on Main Character Screen

On several occasions, including today, when attempting to enter into the game from the main Character Menu screen, “Start Game” is not illuminated. Only option available is “Game Settings”. Only work around I’ve discovered so far is to log back out of the game and then immediately back in. “Start Game” option now appears illuminated and interacts with mouse cursor to allow you to enter into the game itself.

I guess you had selected Public game but you haven’t fulfilled the requirement of that difficulty level.
Edit: add a screenshot

It’s a bug. Hopefully Patch 2.6.9 has fixed it.