Only instance methods can be declared @IBAction But WHY? I'm a new student about Swift

Question

I’m writing my first code in Xcode and this is very basic notepad and I’m trying apply UserDefaults code. But Xcode is asking from me “remove IBAction”. I did. My app has been crashed. Why?

import UIKit

class ViewController: UIViewController {
    
    @IBOutlet weak var GirilenNotTextField: UITextField!
    @IBOutlet weak var GirilenZamanTextField: UITextField!
    @IBOutlet weak var YapilacakIsLabel: UILabel!
    @IBOutlet weak var YapilacakZamanLabel: UILabel!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.
        let KaydedilenNot = UserDefaults.standard.object(forKey: "not")
        let KaydedilenZaman = UserDefaults.standard.object(forKey: "zaman")
        
        if let gelenNot = KaydedilenNot as? String {
            YapilacakIsLabel.text = "Yapilacak Not: \(gelenNot)"
        }
        if let gelenZaman = KaydedilenZaman as? String {
            YapilacakZamanLabel.text = "Yapilacak Zaman: \(gelenZaman)"
        }
        
        @IBAction func KaydetTusunaBasildi(_ sender: Any) {
            UserDefaults.standard.set(YapilacakIsLabel.text!, forKey: "not")
            UserDefaults.standard.set(YapilacakZamanLabel.text!, forKey: "zaman")
            
            YapilacakIsLabel.text="Yapılacak Not: \( GirilenNotTextField.text!)"
            YapilacakZamanLabel.text="Yapılacak Zaman: \( GirilenZamanTextField.text!)"
        }
        
        @IBAction func SilTusunaBasildi(_ sender: Any) {
        }
    }
}