Sorry for the belated response! We’ve been pretty busy gearing up for launch.
If files aren’t being created on your computer, it’s likely a problem with Zwift’s computer permissions. Zwift needs to be able to write to the Documents\Zwift\Activities folder in order to save the .fit files there. If it can’t do that, due to something like permissions or a sync program (like OneDrive), then it won’t be able to upload them either. You can’t upload what you don’t have, right? 
We recommend running Zwift as an Administrator from an Administrator account. If you’re still having problems, don’t forget that you can always submit a ticket via bit.ly/zwiftsupport!