"Undefined Symbol for architecture arm64"

Question

Im new to swift programming. Im building a tic tac toe app for practice. when I want to run the app, the build fails with the error message:

Undefined symbols for architecture arm64:
  "whoToPlay #1 (n: Swift.Int) -> Swift.Bool in Tik_Tak_to.PlayerAction.changeSymbol() -> Swift.String", referenced from:
      Tik_Tak_to.PlayerAction.changeSymbol() -> Swift.String in PlayerAction.o
ld: symbol(s) not found for architecture arm64
clang: error:linker command failed with exit code 1 (use -v to see invocation)

Does somebody know how to fix this? I do this for like 2 weeks and this is my first question here so Im sorry if I forgot to add some information.

struct PlayerAction { 
    var turnNumber: Int = 1
    var players = [Players(player: 1, symbols: "X"), Players(player: 2, symbols: "O")]

    mutating func changeSymbol() -> String {
        if whoToPlay(n: turnNumber) == true {
            let symbolO = players[1].symbols
            turnNumber += 1
            return symbolO
        } else {
            let symbolX = players[0].symbols
            turnNumber += 1
            return symbolX
        }

        func whoToPlay(n: Int) -> Bool {
            if n % 2 == 0 {
                return true         
            } else {
                return false
            }
        }
    }

}

class ViewController: UIViewController {
    
    @IBOutlet weak var A1: UIButton!
    @IBOutlet weak var A2: UIButton!
    @IBOutlet weak var A3: UIButton!
    @IBOutlet weak var B1: UIButton!
    @IBOutlet weak var B2: UIButton!
    @IBOutlet weak var B3: UIButton!
    @IBOutlet weak var C1: UIButton!
    @IBOutlet weak var C2: UIButton!
    @IBOutlet weak var C3: UIButton!
    @IBOutlet weak var turnLabel: UILabel!
    
    var playeraction = PlayerAction()
    
    override func viewDidLoad() {
        super.viewDidLoad()   
    }

    @IBAction func buttonPressed(_ sender: UIButton) {
        if sender.currentTitle == nil {
            sender.setTitle(playeraction.changeSymbol(), for: .normal)
        }
    }
}