I am assuming that your bus has a Allison 3000, since it has the push buttons to shift. Is this a FE or RE?
If it let the engine start, the system sees itself being in neutral. I would start by checking the transmission power, I believe it has a small amperes fuse that power the shift pad also.
it should have a fuse in the electrical panels in the side access.
If you get me the last 7 digits of the vin I can get a wiring diagram