FINAL UPDATE
Good news! The bug is finally fixed and we will be providing full support for Windows 10 Universal (UWP) and Windows 8.1 Universal in v2.7.0, and a performance boost for all Windows Store and Mobile platforms.
UPDATE
We're currently aware of a the following bug when trying to run for Windows Store using the Windows 10 Universal SDK.
The bug appears to be at Unity's end, but we'll keep you updated.
In the meantime, here is the Bug Report.
Here's the error message you may receive:
All the best,Could not load file or assembly 'Windows.Foundation.UniversalApiContract, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null' or one of its dependencies. The system cannot find the file specified.
Joel