In project 23, for the first of three parts to create enemies, the code in the book shows:

    func createEnemy(forceBomb: ForceBomb = .random) {
        let enemy = SKSpriteNode
        enemy = SKSpriteNode()
        var enemyType = Int.random(in: 0...6)
        if forceBomb == .never {
            enemyType = 1
        } else if forceBomb == .always {
            enemyType = 0
        }
        if enemyType == 0 {
           //bomb code here   
        } else {
            enemy = SKSpriteNode(imageNamed: "penguin")
        }
    }

There are two problems with the line

let enemy = SKSpriteNode

First, if you use 'let', then the the line enemy = SKSpriteNode(imageNamed: "penguin"). Second, the line as it is uses the assignment operator '=' but does not instantiate an instance of the SKSpriteNode class with '()' at the end.

The correct code when declaring that variable is:

var enemy: SKSpriteNode!

This makes the variable a 'var' that can be changed. A '!' is used to say that the variable will be defined below for sure.