Convoys appear on the map as a tire in a light blue hexagon. You need to beat 18 of these and you can check your progress on the progress screen in the smartphone. If you are allowed to kill your target the convoy is usually pretty easy. Block the road with a car and when the target gets close, shoot the car with a grenade launcher. Then either kill the remaining enemies or escape.
Some of the later convoys are very difficult, requiring you to knock out two specific enemies with 8-10 enemies shooting at you. The best way I found to do this is to ram the last car in the convoy, then hide and carefully kill the enemies that are not your target, then use cover as available to get close to the targets and knock them out.
If not all of the missions are showing up for you, try progressing in the campaign missions. Scarlet Crusade suggests a certain mission unlocks convoys:
If this doesn't work do some of the fixer missions and hopefully one or the other will unlock the final mission. According to Cian SWEENEY, the 25th fixer mission will unlock a convoy.