You really need to provide us with more specific information. What car, SPECIFICALLY, are you talking about? There are only around 25,000 Skylines in the game, so if we can't figure out what car you're talking about, we can't try to replicate the problem.
Also, when it doesn't install the turbo or let you buy it, what message does it give you? What does it say on your TV screen?
Really, help us help YOU. What you've posted so far is about the equivalent of calling your computer company's tech support and yelling "WHY MY COMPUTER NO WORKY?!"
You have to give us more to work with, and we'll help you figure out what's going on.