If you're like me and the achievement didn't pop after following Redwings05's guide, there's an alternative way to unlock this achievement.
When you have unlocked and completed any barn find (purple circle with a barn and question mark on it), simply drive up to the barn and you will find it has been turned into a "Gift Drop" zone. Press
when prompted to select a car you want to gift and then follow the directions. The achievement should immediately unlock after gifting the car.
If the achievement still doesn't unlock, I would advise waiting until the upcoming hot fix for online connections is released in the coming week as this is (hopefully) what's causing achievement problems.