Are you still running the latest preview build of Windows 10
I'm on 14946. What's wrong with it?
If you have 3rd party antivirus products such as Bitdefender, Kaspersky Antivirus, F-Secure Antivirus and Malwarebytes installed on your PC – your PC might not be able to complete the update to this build and roll-back to the previous build.
My installation always finishes ok. The problem is on Avast side right now.
The problem is with Windows and it not allowing 3rd party AV's to work properly with them in the latest build.
One of the reasons I rolled back till they fix the bugs.