Error 0x803f8001 on Windows 10 Mobile Beta store is a operating system bug. This error pops up when you try to update or download an app or downloads updates to the store. Up until now as of (7/24/2015), there has been no fix from Microsoft.
Many users have attempted soft resets, hard resets and they did not work either so i am not going to suggest this method.
However, one of the users on the Microsoft forum has reported a possible fix which did work for some users. To attempt this fix, follow the procedure below:
1. Change Storage settings to store new apps on the phone, by default it is set to store in the SD CARD.
The above change helped with downloading new apps.
However, Updates to existing apps still encounter the same error (0x803F8001).
A user has reported that “if you used Cortana, told her your name and logged into your account when she asks, then you don’t have a problem logging into email or music etc.”
Further updates, work arounds on this issue; will be posted here.
Check Out https://appuals.com/windows-10-store-error-code-0x803f8001/ If you are experiencing this error on Windows 10 Store whilst downloading apps.