SignIn with Twitter in an iOS application

Question

I'm trying to implement sign in twitter in my app. For getting user details I used the below code. 'logIn' in was working perfectly and I'm receiving userId and userName, but 'loadUser' throwing an error as below.

func loginWithTwitter() {

    TWTRTwitter.sharedInstance().logIn { (session, error) in
        if let error = error {
            print(error)
            return
        }
        if let session = session {
            TWTRAPIClient().loadUser(withID: session.userID, completion: { (user, error) in
                if let error = error {
                    print(error)
                    return
                }
                if let user = user {
                    TWTRAPIClient.withCurrentUser().requestEmail(forCurrentUser: { (email, error) in
                        if let error = error {
                            print(error)
                            return
                        }
                        if let email = email {
                            print(email)
                            print(user.name)
                            print(user.profileImageURL)
                            print(user.screenName)
                        }
                    })
                }
            })
        }
    }

}

Error Domain=TwitterAPIErrorDomain Code=99 "Request failed: forbidden (403)" UserInfo={NSLocalizedFailureReason=Twitter API error : Unable to verify your credentials (code 99), TWTRNetworkingStatusCode=403, NSErrorFailingURLKey=https://api.twitter.com/oauth2/token, NSLocalizedDescription=Request failed: forbidden (403)}

Some documentation says 'OAuth' request is required to get user information from twitter. If it is, please suggest best documentation for OAuth..