This is the 16th mission also known as Episode 16 Traitor's Caravan. You're supposed to follow a convoy and intercept it, but that's quite difficult so there is a trick around it. From the very start of the mission hop on D-Horse, go straight to the airport and hijack the high value cargo. Afterwards, immediately call D-Horse and escape the base.
If you're interested in watching an S Rank walkthrough of the mission, you can view the video below: