String has No Range of String.

Hi, I trying to get my app to run a certain storyboard based on device.

This is my code under DeviceModel.swift file.
import Foundation
import UIKit

public extension UIDevice {

static let modelName: String = {
  var systemInfo = utsname()
  uname(&systemInfo)
  let machineMirror = Mirror(reflecting: systemInfo.machine)
  let identifier = machineMirror.children.reduce("") { identifier, element in
    guard let value = element.value as? Int8, value != 0 else { return identifier }
    return identifier + String(UnicodeScalar(UInt8(value)))
  }

  func mapToDevice(identifier: String) -> String { // swiftlint:disable:this cyclomaticcomplexity
    #if os(iOS)
    switch identifier {
    case "iPod5,1":                 return "iPod touch (5th generation)"
    case "iPod7,1":                 return "iPod touch (6th generation)"
    case "iPod9,1":                 return "iPod touch (7th generation)"
    case "iPhone3,1", "iPhone3,2", "iPhone3,3":   return "iPhone 4"
    case "iPhone4,1":                return "iPhone 4s"
    case "iPhone5,1", "iPhone5,2":         return "iPhone 5"
    case "iPhone5,3", "iPhone5,4":         return "iPhone 5c"
    case "iPhone6,1", "iPhone6,2":         return "iPhone 5s"
    case "iPhone7,2":                return "iPhone 6"
    case "iPhone7,1":                return "iPhone 6 Plus"
    case "iPhone8,1":                return "iPhone 6s"
    case "iPhone8,2":                return "iPhone 6s Plus"
    case "iPhone8,4":                return "iPhone SE"
    case "iPhone9,1", "iPhone9,3":         return "iPhone 7"
    case "iPhone9,2", "iPhone9,4":         return "iPhone 7 Plus"
      case "iPhone10,1", "iPhone10,4":        return "iPhone 8"
      case "iPhone10,2", "iPhone10,5":        return "iPhone 8 Plus"
      case "iPhone10,3", "iPhone10,6":        return "iPhone X"
      case "iPhone11,2":               return "iPhone XS"
      case "iPhone11,4", "iPhone11,6":        return "iPhone XS Max"
      case "iPhone11,8":               return "iPhone XR"
      case "iPhone12,1":               return "iPhone 11"
      case "iPhone12,3":               return "iPhone 11 Pro"
      case "iPhone12,5":               return "iPhone 11 Pro Max"
      case "iPhone12,8":               return "iPhone SE (2nd generation)"
      case "iPad2,1", "iPad2,2", "iPad2,3", "iPad2,4":return "iPad 2"
      case "iPad3,1", "iPad3,2", "iPad3,3":      return "iPad (3rd generation)"
      case "iPad3,4", "iPad3,5", "iPad3,6":      return "iPad (4th generation)"
      case "iPad6,11", "iPad6,12":          return "iPad (5th generation)"
      case "iPad7,5", "iPad7,6":           return "iPad (6th generation)"
      case "iPad7,11", "iPad7,12":          return "iPad (7th generation)"
      case "iPad4,1", "iPad4,2", "iPad4,3":      return "iPad Air"
      case "iPad5,3", "iPad5,4":           return "iPad Air 2"
      case "iPad11,4", "iPad11,5":          return "iPad Air (3rd generation)"
      case "iPad2,5", "iPad2,6", "iPad2,7":      return "iPad mini"
      case "iPad4,4", "iPad4,5", "iPad4,6":      return "iPad mini 2"
      case "iPad4,7", "iPad4,8", "iPad4,9":      return "iPad mini 3"
      case "iPad5,1", "iPad5,2":           return "iPad mini 4"
      case "iPad11,1", "iPad11,2":          return "iPad mini (5th generation)"
      case "iPad6,3", "iPad6,4":           return "iPad Pro (9.7-inch)"
      case "iPad7,3", "iPad7,4":           return "iPad Pro (10.5-inch)"
      case "iPad8,1", "iPad8,2", "iPad8,3", "iPad8,4":return "iPad Pro (11-inch) (1st generation)"
      case "iPad8,9", "iPad8,10":           return "iPad Pro (11-inch) (2nd generation)"
      case "iPad6,7", "iPad6,8":           return "iPad Pro (12.9-inch) (1st generation)"
      case "iPad7,1", "iPad7,2":           return "iPad Pro (12.9-inch) (2nd generation)"
      case "iPad8,5", "iPad8,6", "iPad8,7", "iPad8,8":return "iPad Pro (12.9-inch) (3rd generation)"
      case "iPad8,11", "iPad8,12":          return "iPad Pro (12.9-inch) (4th generation)"
      case "AppleTV5,3":               return "Apple TV"
      case "AppleTV6,2":               return "Apple TV 4K"
      case "AudioAccessory1,1":            return "HomePod"
      case "i386", "x8664":             return "Simulator \(mapToDevice(identifier: ProcessInfo().environment["SIMULATORMODELIDENTIFIER"] ?? "iOS"))"
      default:                    return identifier
      }
      #elseif os(tvOS)
      switch identifier {
      case "AppleTV5,3": return "Apple TV 4"
      case "AppleTV6,2": return "Apple TV 4K"
      case "i386", "x8664": return "Simulator \(mapToDevice(identifier: ProcessInfo().environment["SIMULATORMODELIDENTIFIER"] ?? "tvOS"))"
      default: return identifier
      }
      #endif
    }

    return mapToDevice(identifier: identifier)
  }()

}

This is my code under the appDelegate

func application( application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
    var storyboard: UIStoryboard

    // instantiate the UIWindow
    self.window = UIWindow(frame: UIScreen.main.bounds)

    // Get the model name based in the extension.
    let modelName = UIDevice.modelName

    // If the modelName variable contains the string "iPhone 6" inside.
    if (modelName.rangeOfString("iPad Air 2") != nil) {
      storyboard = UIStoryboard(name: "mainStoryInVC", bundle: nil)
      self.window!.rootViewController = storyboard.instantiateInitialViewController() as!ViewController
    }
    else if (modelName.rangeOfString("iPad Pro (9.7-inch)") != nil) {
      storyboard = UIStoryboard(name: "mainStoryInVC", bundle: nil)
      self.window!.rootViewController = storyboard.instantiateInitialViewController() as! ViewController
    }

    self.window!.makeKeyAndVisible()

    return true
  }

It give me a problem that modelName is a string that has no value rangeOfString. It's not giving me an option to replace it. How do I fix this?

Code Block 
        if modelName.range(of: "iPad Air 2") != nil {
            storyboard = UIStoryboard(name: "mainStoryInVC", bundle: nil)
            self.window!.rootViewController = storyboard.instantiateInitialViewController() as!ViewController
        } else if modelName.range(of: "iPad Pro (9.7-inch)") != nil {
            storyboard = UIStoryboard(name: "mainStoryInVC", bundle: nil)
            self.window!.rootViewController = storyboard.instantiateInitialViewController() as! ViewController
        }

Code Block 
        if modelName.contains("iPad Air 2") {
            storyboard = UIStoryboard(name: "mainStoryInVC", bundle: nil)
            self.window!.rootViewController = storyboard.instantiateInitialViewController() as!ViewController
        } else if modelName.contains("iPad Pro (9.7-inch)") {
            storyboard = UIStoryboard(name: "mainStoryInVC", bundle: nil)
            self.window!.rootViewController = storyboard.instantiateInitialViewController() as! ViewController
        }