This Bug was one of the things the devs specifically mentioned being addressed with Today's update. try again in a few hours after installing the update.

Keep trying. This is a know bug with no easy fix, AFAK.

Play it via cloud streaming, then go back to your Xbox. This will force a save resync which may solve the issue. That's what worked for me.

I'm trying to do the weekly forzathon with the VW type 2 bus. I've tried my already owned one, bought a new one and didn't modify it at all. Tried driving around. I also restarted the game and my Xbox.  Anyone else have an idea on how to get it to work?

Can't progress on weekly forzathon