Index -> Appendix -> Can't
If one piece of game text says you "can't" do something that a different piece of game text would normally allow, the "can't" takes precedence.
Part of Harlan's game text reads: "Stop Harlan -- Play a support character at normal cost, even if you already control a character who has the same title." Fall of Earth's game text reads: "You can't play support characters." While Fall of Earth is the current mission, you can't use Harlan's game text to play a support character.