It's a modern ritual to pre-load a game in the lead up to its release, in order to ensure you can play the game as soon as it launches. Pre-loading is available for Forza Horizon 3, and if you've purchased the game then you'll be able to start downloading it from the Windows Store right now, assuming you've upgraded Windows 10 to Anniversary Edition.
Well, that's what's meant to happen, anyway. Some users are reporting that their pre-loads aren't working, and several people have taken to the Turn 10 forums to report the error. If you belong in that unfortunate camp, rest assured that someone is looking at it.
According to a spokesperson from Turn 10, writing in the Forza Horizon 3 forums, it's being looked at. "We've got a strike team from the Windows Store helping us investigate these errors as I type this: both the pause and resume/restart download errors as well as the 0x80072EE7 and 0x80070005 error codes."