The combination of VS2015 and 15063 is not supported. I suspect you have an older Windows SDK installed alongside? It looks like what’s happening is that VS2015 is falling back to that old version and picking up a version of the d3d12 header that’s incompatible with the latest d3dx12.h.
To solve, you can either move to VS2017, or install a 3rd(!) version of the Windows SDK that’s new enough to contain the missing definitions but no newer than 14193 (which is the latest with VS2015 support).