BuildConfig.VERSION_CODE is missing after update to ‘com.android.tools.build:gradle:4.1.0’

add buildConfigField to buildTypes in module build.gradle file

buildConfigField("long", "VERSION_CODE", "${defaultConfig.versionCode}")        
buildConfigField("String","VERSION_NAME","\"${defaultConfig.versionName}\"")

example,

buildTypes {
    debug{
        buildConfigField("long", "VERSION_CODE", "${defaultConfig.versionCode}")
        buildConfigField("String","VERSION_NAME","\"${defaultConfig.versionName}\"")

        //..
    }
    release {
        buildConfigField("long", "VERSION_CODE", "${defaultConfig.versionCode}")
        buildConfigField("String","VERSION_NAME","\"${defaultConfig.versionName}\"")

        //...
    }
}

the defaultConfig.versionCode and defaultConfig.versionName value is

defaultConfig {
    minSdkVersion 16
    targetSdkVersion 29
    versionCode 1
    versionName "1.0"

    //..
}

in your module build.gradle

you can create field in BuildConfig by writing buildConfigField method in module build.gradle. The method first parameter is create field type, second is field name, third is value.

Android Studio 4.1