如何修复在范围内找不到“FirebaseApp”？

我有问题。请帮忙。每当我尝试运行此代码时，它都会出现错误并显示在范围内找不到“FirebaseApp”。请帮忙！！！

import UIKit
import Firebase
@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {
var window: UIWindow?
func application(_ application: UIApplication,
didFinishLaunchingWithOptions launchOptions:
[UIApplicationLaunchOptionsKey: Any]?) -> Bool {
FirebaseApp.configure()
return true
}