Make sure your running the batch file/devcon as an administrator. If that doesn't work, make sure you have the 64 bit version of devcon if you're running 64 bit windows, and the 32 bit version of devcon if you're running 32 bit windows.

Side note for people who might search this in the future:

The Asus Z270-A WILL NOT RUN 7 Vega. At least not without additional hardware. Its impossible to install the drivers, they'll crash every time no matter how you do it. 6 cards is doable, as long as they're placed in every slot EXCEPT PCIeX16_1. If a card is placed in the first x16 slot, then the drivers will crash while installing if you try to use more than 5 cards. Just a heads up for anyone who might be having this problem.