I have finally found the answer to this problem! Go to manage game like people have said and go to saved data, but instead of pressing”delete all” press on the account you want to delete the save from and click”delete everywhere.” Trust me, it works because I was trying to find this myself.
Hey guys,
So I don't know how useful it will be at this point in time but after playing Odyssey I decided to replay the entire franchise from the start and I just started Unity today. Of course I got the same problem, couldn't start a new game. Btw if somebody already posted this sorry I didn't go through the whole thread
So here's how I got around it.
1. Put your Xbox offline (System>Network>Go offline)
2. Delete Save Data (Manage games>Saved Data>Delete all)
3. Still offline start a new game and wait until you see the game saving during the first cinematic (not the helix fake advert but the one set in france viewing a castle)
4. Check that there is a new saved data on your Xbox (should be much smaller than the old one)
5. Close the game
6. Go back online and start the game again
7. You'll have a message talking about conflicting data, choose the newest et voilà ^^ NEW GAME ^^
Happy revolution everyone ;P