So, I created sandbox user account. Here you can do this. Try to create if after create button nothing happens - open your browser's console and check servers' respond. You will see the 500 error, also you, find respons' JSON (in console) and you will see validationResult object. Open it and you will see whats' cause the error. In my case it was "This password is too simple and can be easily guessed", so I change the password and voila - I have new sandbox user.

Swift global: extension UINavigationController{   open override func viewDidLoad() {     super.viewDidLoad()     if #available(iOS 13, *) {       self.navigationBar.standardAppearance = UINavigationBarAppearance()       self.navigationBar.standardAppearance.configureWithDefaultBackground()       self.navigationBar.standardAppearance.backgroundColor = mainColor       self.navigationBar.standardAppearance.titleTextAttributes = [NSAttributedString.Key.foregroundColor: UIColor.white]     }     else {       self.navigationBar.titleTextAttributes = [NSAttributedString.Key.foregroundColor: UIColor.white]       self.navigationBar.barTintColor = mainColor     }   } }