빨간색 글씨때문에 막혀요. 그리고 Cloud Firestore 초기화 코드가 달라요.

val user =User(uid, username.text.toString())
에서 username부분이 빨간색이 떠요.

val db = FirebaseFirestore.getInstance.collection("users")

에서 FirebaseFirestore가 빨간색이 떠요.

addOnSuccessListener{
    Log.d(TAG, "데이터베이스 성공")
}
addOnFailureListener{
    Log.d(TAG, "데이터베이스 실패")
}
에서

addOnSuccessListener와 addOnFailureListener가 빨간색이 떠요.
multiDexEnabled true와 "com.android.support:multidex:1.0.3"은 어디서 나온건가요?
어디서 복사하기 붙여넣기 했나요? 파이어스토어 데이터베이스에서 검색해봐도 똑같은
코드가 없습니다. 어떻게 했는지 모르겠어요.
package com.example.chatting_video

import android.content.Intent
import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import com.google.firebase.auth.FirebaseAuth
import kotlinx.android.synthetic.main.activity_main.*
import android.util.Log as Log
import kotlinx.android.synthetic.main.activity_main.login_button_main as login_button_main


class MainActivity : AppCompatActivity() {

private lateinit var auth: FirebaseAuth// ...

 private val TAG: String = MainActivity::class.java.simpleName


 override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)"


auth = FirebaseAuth.getInstance()

join_button.setOnClickListener {

 val email = email_area.text.toString()
val password = password_area.text.toString()
auth.createUserWithEmailAndPassword(email, password)
.addOnCompleteListener(this) { task ->
 if (task.isSuccessful) {
// Sign in success, update UI with the signed-in user's information

 Log.d(TAG, "성공")

val uid = FirebaseAuth.getInstance().uid



 val user =User(uid, username.text.toString())

// 여기에서 데이터베이스에 넣음
 val db = FirebaseFirestore.getInstance.collection("users")
db.document(uid)
.set(user)
addOnSuccessListener{
 Log.d(TAG, "데이터베이스 성공")
}
 addOnFailureListener{
 Log.d(TAG, "데이터베이스 실패")
}



 val intent = Intent(this, ChatListActivity::class.java)
intent.flags = Intent.FLAG_ACTIVITY_NEW_TASK or Intent.FLAG_ACTIVITY_CLEAR_TASK
 startActivity(intent)


} else {

Log.d(TAG, "실패", task.exception)


}
}
 }




 login_button_main.setOnClickListener {

 val intent = Intent(this, LoginActivity::class.java)
startActivity(intent)
}



 }


}

plugins {
    id 'com.android.application'
    id 'kotlin-android'
    id 'kotlin-android-extensions'
}
apply plugin: 'com.google.gms.google-services'

android {
    compileSdkVersion 30
    buildToolsVersion "30.0.3"

    defaultConfig {
        applicationId "com.example.chatting_video"
        minSdkVersion 16
        targetSdkVersion 30
        versionCode 1
        versionName "1.0"

        testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
        multiDexEnabled true
    }

    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
        }
    }
    compileOptions {
        sourceCompatibility JavaVersion.VERSION_1_8
        targetCompatibility JavaVersion.VERSION_1_8
    }
    kotlinOptions {
        jvmTarget = '1.8'
    }
}

dependencies {

    implementation "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
    implementation 'androidx.core:core-ktx:1.2.0'
    implementation 'androidx.appcompat:appcompat:1.1.0'
    implementation 'com.google.android.material:material:1.1.0'
    implementation 'androidx.constraintlayout:constraintlayout:1.1.3'
    implementation 'com.google.firebase:firebase-auth:19.3.1'
    testImplementation 'junit:junit:4.+'
    androidTestImplementation 'androidx.test.ext:junit:1.1.1'
    androidTestImplementation 'androidx.test.espresso:espresso-core:3.2.0'
    implementation platform('com.google.firebase:firebase-bom:27.1.0')
    implementation 'com.google.firebase:firebase-analytics'
    implementation platform('com.google.firebase:firebase-bom:26.8.0')


}

// Create a new user with a first and last name
val user = hashMapOf(
        "first" to "Ada",
        "last" to "Lovelace",
        "born" to 1815
)

// Add a new document with a generated ID
db.collection("users")
    .add(user)
    .addOnSuccessListener { documentReference ->
        Log.d(TAG, "DocumentSnapshot added with ID: ${documentReference.id}")
    }
    .addOnFailureListener { e ->
        Log.w(TAG, "Error adding document", e)
    }

https://firebase.google.com/docs/firestore/quickstart