compileSdkVersion is not specified error in React Native Android

I get the build error when I try to build an Android app in my React Native project.

When I run react-native run-android, it says:

> Configure project :react-native-os
WARNING: Configuration 'compile' is obsolete and has been replaced with 'implementation' and 'api'.
It will be removed at the end of 2018. For more information see: http://d.android.com/r/tools/update-dependency-configurations.html

FAILURE: Build failed with an exception.

* What went wrong:
A problem occurred configuring project ':react-native-os'.
> compileSdkVersion is not specified.

In Android Studio, I can see that it says the same thing: Cause: compileSdkVersion is not specified.

So I edited build.gradle(Module: app) file in Android Studio, but the problem persists and the error message doesn't change.

android {
//    compileSdkVersion rootProject.ext.compileSdkVersion
    compileSdkVersion 28
    buildToolsVersion "28.0.3"
    compileOptions {
        sourceCompatibility JavaVersion.VERSION_1_8
        targetCompatibility JavaVersion.VERSION_1_8
    }

I tried solutions in other threads like this one but nothing worked. Does anyone know what the problem is here?