I have noticed that once in a while (I think maybe with just this particular mission, but I'm not sure), the VAB doesn't print the rocket/booster combo in a different color to show it can't lift the current payload. It might be that it misses it when the payload is just a little too heavy for the rocket combo, which only occurs in a few rare occasions such as a lunar pass with the XMS-2, a B-Kicker, and a boosted Titan. In the past this has led me to accidentally autopurchase the rocketry for a mission I can't fly, since the eye focuses on the color of the rocket/booster down below, and not the current payload up at the top.

(In case anyone needs a quick screenshot for comparison, this is how the rocket/booster combo should look at the bottom of the screen:)