I ran into the same problem, probably also in the same book. This worked for me: in the document outline I selected not the Cell, but some content of the Cell (Label, for example), and then I tried to open the Assistant. ViewController opened, but when you click the Automatic option above, two options are provided - click the Cell.swift and there you go. I hope this helps!
Moni11
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Adding this code to the last line worked for me:
PlaygroundPage.current.liveView = MKMapView(frame: CGRect(x: 0, y: 0, width: 400, height: 800))