This product is not intended to be used in conjunction with the OnlineSubsystemSteam module.
By purchasing any 2 of the 3 light editions (Single Player / Multi-Player / Subsystem) you may request access to the full edition, therefore obtaining the third one for free! The purchases must be made on the same account. This is not an instant process.
(-TEMPORARY-) In 4.20.0, Epic has hard-coded their built-in subsystems, therefore making custom subsystems impossible to register without modifying the engine. You can read more about this issue over here. Until this is fixed, only the NetDriver, the Core/Web modules and the example project will work work out of the box.