"use the garage to install the upgrades for the jack"

[Kerous]

what does this mean?

[jack_of_tears]

You have multiple builds you can create for your car. Go into the garage and choose the build which makes up the car you're supposed to race with. The pieces you need should be marked with a little green dot - they will not be all the best pieces you have on offer.

[StridΣR]

That means you got to press ESC and click on GARAGE, then you can install upgrades

[Xerany]

If the above two weren't clear enough, let me explain things bare-bones.

There are vehicles in this game by the name of Archangels. Basically, they're preset vehicles, built to best fit a certain thing. The best car for speed, the best car for ramming, the best car for side-ramming and grinding, etc.

For the quest you're in, you need one of these Archangels; the Jack. Enter your garage and look at the bottom of the UI, which should show you a button to press to look at these Archangels. Select the Jack and see which parts it needs to 'become' the Jack, and you can continue with the quest! :D

[Mr. Paintwolf]

Once you hit "ESC" next to your peronal upgrades, labled under "Max" you'll see "Garage" That's where you customise and upgrade your ramming bars, armor, tires, harpoon, THUNDERpoons, ect.