Late night mistake. Accepted the (last) Hurk mission to use Buzzer to Takedown the truck from the air. Did not think that I didn't actually have the Takedown ability yet. Now seem to be stuck with this mission and do not see a way to fully cancel and get back to finish earning this in the Kyrat races.(had one completed but need the second one completed) Help? Thanks fellow gamers. Lesson learned: Don't pull an all-nighter without actually reading the mission details
BruiserlaRue
It wouldn't let me cancel the mission either and like you this was before I had unlocked the vehicle take down ability
What you must do:
1. Return to the area with the buzzard, you'll know you're there once you see *check point* onscreen
2. now you can cancel the mission and retry it later (for reasons unknown you cannot cancel the mission until you reach the first check point)
3. complete one of each kyrat race type
the mission is impossible to beat without the vehicle take down skill because you have to hijack the truck by jumping out of the gyro copter.