Hi Bill,
Don't feel stupid, it's a perfectly reasonable and logical question considering you encountered this 'error' like that.
I don't know all the mission names by heart, is this a stock mission, or a custom mission?
If the latter is true, then it could be that the mission creator included one or more addon ships, from the New Horizons package, or downloaded from the Shipyard. If you do not have it/them installed, the mission can't be played.
But if it is a mission that came with the game you have installed, then it shouldn't give you such an error. Not uless there's something strange going on.
Regards,
Fred