エラー：Unresolved referenceが発生してしまう。

前提・実現したいこと

android studioで、「はじめてのandroidプログラミング　第４版」chapter５「最強！？じゃんけんアプリ」を作っております。

発生している問題・エラーメッセージ

本の記述通り書いているのにもかかわらず、設定したIDや定数等に対して、
以下のようなUnresolved referenceというエラーが発生してしまいます。

Unresolved reference: gu
Unresolved reference: it
Unresolved reference: choki
Unresolved reference: it
Unresolved reference: pa
Unresolved reference: it
Unresolved reference: myHandImage
Unresolved reference: gu
Unresolved reference: myHandImage
Unresolved reference: choki
Unresolved reference: myHandImage
Unresolved reference: pa
Unresolved reference: gu
Unresolved reference: gu
Unresolved reference: comHandImage
Unresolved reference: choki
Unresolved reference: comHandImage
Unresolved reference: pa
Unresolved reference: comHandImage
Unresolved reference: resultLavel
Unresolved reference: resultLavel
Unresolved reference: result_win
Unresolved reference: resultLavel
Unresolved reference: backButton

初心者のため、何が悪いのかが分からず困惑しております。
ご教授の程、どうぞよろしくお願い致します。

該当のソースコード

MainActivity
1package com.example.kotolinfight
2
3import android.content.Intent
4import android.os.Bundle
5import android.view.View
6import androidx.appcompat.app.AppCompatActivity
7
8class MainActivity : AppCompatActivity() {
9    override fun onCreate(savedInstanceState: Bundle?) {
10        super.onCreate(savedInstanceState)
11        setContentView(R.layout.activity_main)
12
13        gu.setOnClickListener { onJankenButtonTapped(it)}
14        choki.setOnClickListener { onJankenButtonTapped(it)}
15        pa.setOnClickListener { onJankenButtonTapped(it)}
16
17        }
18
19    fun onJankenButtonTapped(view: View?){
20        val intent = Intent(this, ResultActivity::class.java)
21        intent.putExtra("MY_HAND",view?.id)
22        startActivity(intent)
23    }
24}

ResultActivity
1package com.example.kotolinfight
2
3import androidx.appcompat.app.AppCompatActivity
4import android.os.Bundle
5
6class ResultActivity : AppCompatActivity() {
7    override fun onCreate(savedInstanceState: Bundle?) {
8        super.onCreate(savedInstanceState)
9        setContentView(R.layout.activity_result)
10        val id=intent.getIntExtra("MY_HAND",0)
11
12        val myHand: Int
13        myHand= when(id){
14            R.id.gu-> {
15                myHandImage.setImageResource(R.drawable.gu)
16                gu
17            }
18            R.id.choki-> {
19                myHandImage.setImageResource(R.drawable.choki)
20                choki
21            }
22            R.id.pa->{
23                 myHandImage.setImageResource(R.drawable.pa)
24                 pa
25            }
26            else->gu
27        }
28
29        val comHand=(Math.random()*3).toInt()
30        when(comHand){
31            gu->comHandImage.setImageResorce(R.drawable.com_gu)
32            choki->comHandImage.setImageResorce(R.drawable.com_choki)
33            pa->comHandImage.setImageResorce(R.drawable.com_pa)
34        }
35
36
37        val gameResult=(comHand-myHand+3)%3
38
39        when(gameResult){
40            0->resultLavel.setText(R.string.result_draw)
41            1->resultLavel.setText(R.string.result_win)
42            2->resultLavel.setText(R.string.result_lose)
43
44        }
45        backButton.setOnClickListener{finish()}
46    }
47}
48

gradle(プロジェクト)です。↓

gradle(project）
1// Top-level build file where you can add configuration options common to all sub-projects/modules.
2buildscript {
3    ext.kotlin_version = "1.4.30"
4    repositories {
5        google()
6        jcenter()
7    }
8    dependencies {
9        def lifecycle_version = "2.2.0"
10        classpath "com.android.tools.build:gradle:4.1.2"
11        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
12
13        // NOTE: Do not place your application dependencies here; they belong
14        // in the individual module build.gradle files
15    }
16}
17
18allprojects {
19    repositories {
20        google()
21        jcenter()
22    }
23}
24
25task clean(type: Delete) {
26    delete rootProject.buildDir
27}

gradle(モジュール)です。↓

gradle(module）
1plugins {
2    id 'com.android.application'
3    id 'kotlin-android'
4}
5
6android {
7    compileSdkVersion 30
8    buildToolsVersion "30.0.3"
9
10    defaultConfig {
11        applicationId "com.example.kotolinfight"
12        minSdkVersion 19
13        targetSdkVersion 30
14        versionCode 1
15        versionName "1.0"
16
17        testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
18    }
19
20    buildTypes {
21        release {
22            minifyEnabled false
23            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
24        }
25    }
26    compileOptions {
27        sourceCompatibility JavaVersion.VERSION_1_8
28        targetCompatibility JavaVersion.VERSION_1_8
29    }
30    kotlinOptions {
31        jvmTarget = '1.8'
32    }
33}
34
35dependencies {
36
37    implementation "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
38    implementation 'androidx.core:core-ktx:1.3.2'
39    implementation 'androidx.appcompat:appcompat:1.2.0'
40    implementation 'com.google.android.material:material:1.3.0'
41    implementation 'androidx.constraintlayout:constraintlayout:2.0.4'
42    testImplementation 'junit:junit:4.+'
43    androidTestImplementation 'androidx.test.ext:junit:1.1.2'
44    androidTestImplementation 'androidx.test.espresso:espresso-core:3.3.0'
45}