After you have launched the mod, buy a car in the showroom and immediately exit the Payback game, then go back into the game and you will have a Nissan GTR Premium. But you need to buy a car from a racing dealer, or rather just pick it up, because its cost is zero.
Look for the FrostyModManager v188.8.131.52 program in the archive with the car you download. So, with its help, proceed to install the Nissan GTR Premium mod in NFS Payback, there are also instructions there.
It looks like the car model was transferred from Need For Speed 2015 to the current one, like a mod for Nissan Premium 2017. It looks like a new addition to the fleet of a racing Dealer.