I have played with all the other cars and have won with all of them. I haven't unlocked the last one for the achievement. Why is this? I have won multiple times and would like to know what else I can do.
The last car you are trying to unlock is called "Sweet Tooth", which is a PS4 exclusive vehicle.
You need to complete at least one game with all cars except the Merc and the Venom. These games can be played online or offline.
As mentioned in this answer: https://gaming.stackexchange.com/a/227959/117990