As Part 1: Top-level Architecture reads:

As we have seen in the previous chapters, to create a top-level actor one must call System.ActorOf().

This method doesn't seem to be mentioned in any of the previous chapters of the doucmentation as visible on https://getakka.net (e.g. 'What is Akka.NET?', 'What are actors?' or 'Akka.NET Libraries and Modules').

I had to browse the API docs to learn how to create an instance of ActorSystem with ActorSystem.Create. These tutorials should cover this step.