The Allison MD3060 transmission is programmed such that it will not go into gear if the engine revs are above 700 rpm.   Since Cummins recommends that the engine not be idled for more than five minutes on "slow" idle, many folks implement the "fast idle" feature (by turning on the cruise control and tapping the set speed button) to allow the engine to warm up and air pressure to build up faster.   However, since this sets the idle speed above 700 rpm, if you forget to cancel the "fast idle" and attempt to shift the transmission into gear, the transmission pad will flash "6" and will remain in neutral.

NOTE: This is related to the accelerator cutout problem.