If you are stuck at 97% and sure you competed all the Exibition races you will need to follow these steps
•Disconnect your wifi or Ethernet cable from your Xbox my (not go offline)
•Go to Forza Horizon 3s Manage game and delete the save file.
•Start a new game, play offline still and unlock the races you may need. I was at 97% and completed two exhibitions to be sure.
•Onve you have completed the races needed, pause the game and dashboard to the Xbox main menu. Go to manage game and delete the hard save file again.
•With Forza Horzion still running and the save file deleted, connect your wifi or Ethernet cable and the achievement will pop.
(Keep In Mind that this will get you the achievement but may not not progress your game.)
I hope this helps!