Ouch. This is bug of current incompatibility with InreadiBuild. It would be fixed in the future, but today there is a workaround.
Engine\Source\Programs\UnrealBuildTool\Platform\HoloLens\UEBuildHoloLens.cs
in HoloLens add the following method
public override bool CanUseXGE()
{
return false;
}