Hello. I bought and installed the Nurburgring booster pack an hour ago, but when I go into the buy cars page and try to buy one of the cars, it says it is not installed. I tried uninstalling and reinstalling it. Halp
Completely shutdown your console by holding the Xbox One’s power button (not the controller) until it turns off. Then, retry.
I tried that and it still says installing content (permanently at 5%)
How long have you waited for it to install?
It has been over an hour now.
Your Xbox One has a solid internet connection?
Yes.
If it insists on not installing, your next course of action would to be to contact Xbox Support. Turn 10 does not have authority over Marketplace transactions.
Have you also tried removing the install from the Apps/Games-queue, powercycle the console using the CORRECT power-button? If this fails, do you have the means/possibility to hook up the console DIRECTLY to the modem using a cable (without any WiFi, intermediate switches / routers)?