Cannot resolve R in andorid studio

Question

Application is installing fine and the build is okay. But in the whole app I can see this error. It happened when I tried to upgrade the project from 4.2.2 to 7.2. But it failed and rolled back. I have done the following steps.

1. clean the project.
2. Rebuild the project.
3. Sync with gradle files.
4. Invalidate cache and restart.
5. Moved the project to other dir and run it from there.

also I have read other answers about cannot resolving R. cannot resolve R

Here is my app project level gradle

buildscript {
repositories {
    google()
    mavenCentral()
}
dependencies {
    classpath 'com.android.tools.build:gradle:4.2.2'
    classpath 'com.google.gms:google-services:4.3.15'

    // Add the Crashlytics Gradle plugin (be sure to add version
    // 2.0.0 or later if you built your app with Android Studio 4.1).
    classpath 'com.google.firebase:firebase-crashlytics-gradle:2.5.2'

    // NOTE: Do not place your application dependencies here; they belong
    // in the individual module build.gradle files
}
  }



 allprojects {
    repositories {
        google()
        mavenCentral()
        centre()
     }
   }