Unable to get alerts to show on simulator screen

I am currently working on a social media app and so far I have been able to make a LoginViewController & a SignUpVIewController. Right now I am currently trying to have it where on the SignUpViewController if not all required fields are filled out then the app will display an alert. However, every time i run the app the alert doesn't show up on the simulator screen. Same goes for Xcode's terminal.

My code for the SignUpViewController is as follows:

 
import UIKit
import Parse
 
class SignUpViewController: UIViewController
{
  @IBOutlet weak var emailTextField: UITextField!
   
  @IBOutlet weak var usernameTextField: UITextField!
   
  @IBOutlet weak var passwordTextField: UITextField!
   
  @IBOutlet weak var confirmPasswordFieldText: UITextField!
   
  @IBOutlet weak var signUpButton: UIButton!
   
   
  override func viewDidLoad() {
    super.viewDidLoad()
  }
   
   
  @IBAction func signUpButtonTapped(_ sender: UIButton)
  {
    let username = usernameTextField.text
    let password = passwordTextField.text
    let email = emailTextField.text
    let confirmpw = confirmPasswordFieldText.text
     
    if username == "" || password == "" || email == "" || confirmpw == ""
    {
      let alert = UIAlertController(title: "Error", message: "Please fill out all required fields", preferredStyle: .alert)
      alert.addAction(UIAlertAction(title: "OK", style: .default,handler: nil))
      present(alert, animated: true, completion: nil)
      return
    }
     
    let newUser = User(username: username!, password: password!, email: email!)
  }
   
   
}
   
class User
{
  var username: String
  var password: String
  var email: String
   
  init(username: String, password: String, email: String) {
    self.username = username
    self.password = password
    self.email = email
  }
}

All the buttons are connected to the correct spots. Would greatly appreciate if anyone can help me fix this.