Hello alex313962,
I seem to of found the problem and luckily it’s very simple. The line you added to the alpha.build.cs needs to be edited. This is the line:
DynamicallyLoadedModuleNames.add("OnlineSubsystemSteam");
The “a” in “add” needs to be capitalized. Alternatively, you can leave this line commented out and uncomment the section that we mentioned earlier. It should build correctly after that.