Try holding the button for five seconds. I’ve also had it not allow takeoff config when the cabin isn’t yet ready - I’m not sure this is correct behaviour.
Sim just crashed flying KDFW to LFPG - no error in the event viewer, just closed down. No other aircraft does this in the sim. So frustrating, third long haul attempt with v1.0.9 and third crash - I had a usable aircraft prior to this update!