@VerMishelb: The reason why it’s disabled is that when your destination lies in another constellation, you need to first explore both “gateway” star systems which “connect” the constellations together (i.e., the star system where you leave one constellation and the star system you enter the next one). I have now fixed this to allow a single hop across “unexplored” route before complaining. Fixed in v.23 Bug.
@GgWw1175: I’ve changed the wording to avoid mentioning highways. I’ve also changed the color of the line as you jump across constellations to make it obvious that it’s not part of either constellation. Fixed in v.23 Bug.