Actual source of issue:
warning C4996: ‘GetBuildIdAsSteamKey’: UE4_PROJECT_STEAMGAMEDESC has been renamed to UE_PROJECT_STEAMGAMEDESC. Please update your code to the new API before upgrading to the next release, otherwise your project will no longer compile.
This is a lie. You must change these to UE_PROJECT*Whatever in your target.cs or it will not compile.