This could be down to one of two things.
Firstly, if you're using a car dealer or non-DarkRP gamemode, then either required variables aren't set, or default hooks aren't called either.
Secondly, the vehicle you're trying to use may not be setup. Check the supported vehicles tab to see which vehicles are currently supported.
Once you've done that, feel free to open support ticket, and we'll do our best to help.
