Thanks for the screenshot. The code does show as completed on your account.
If you haven't already, try these steps:
- Fully close Forza Horizon 6 and reopen it.
- Make sure you are signed into the same Xbox account shown in the order history.
- Check the in-game Message Center, Gifts, or Garage section.
- Try driving to a Festival site or house, since some reward cars only appear after syncing online.
If it still does not appear, was this code from a promo/event code such as Fanta? Some promo cars can take a little time to sync into the game.