**As of October 6, 2015, this achievement is working as intended. If you were at 96% before the patch, do a quick multiplayer race and it should unlock. Most are reporting that it works after doing a quick race at Laguna Seca.**
There are a total of 26 locations (or "environments") in Forza 6. For this achievement, you just need to complete one race at every location, not all of the routes. For instance, for Daytona, you can just do a race on the big oval track, and that's it. Same for any other location with multiple variations.
By far the easiest way to get this is to goto Multiplayer and hit Y to start a Private Lobby. From there, go to the Game Options and change the race type to a Timed Race, as well as setting the time limit to 00:30. Pick whatever car you want (or work on the Ferraris for the Car Culture achievement simultaneously), add a drivatar, and start the race.
You don't even have to move. Once the thirty second timer is up, the race is over and you get credit for racing at that location. Move on to the next location, rinse and repeat.