You may found that the Beta Store (gray icon) on Windows 10 Mobile has this error: 0x803F8001. See how the Insider describe it in detail.

How to fix Error 0x803F8001 Windows 10 Mobile
How to fix Error 0x803F8001 Windows 10 Mobile?

I’ve got beta store error 0x803F8001 on Windows 10 Mobile. Beta store finds some updates and can’t install it. It finds updates for programs installed via classic store, and those apps can’t be started (I’ve got dark icon on start screen. There is 21 updates in Beta store in total.
How to fix it? See the instruction from Windows 10 Support Engineer below:
To fix the issue with updates in store beta, you may refer to the link given below to perform soft reset of your phone and check if the issue persists.
If you have any further issue, please let us know. We will try our best to support you on Windows 10.

Update October 17, 2016
As advised from MIKE D (an Insider), his solution is sign out of your Microsoft Account (Windows Store) and sign back in again then try downloading the App