Your app is using an unsupported Gradle project. To fix this problem,

need help. Flutter throws error when creating apk

[!] Your app is using an unsupported Gradle project. To fix this problem, create a new project by running flutter create -t app <app-directory> and then move the dart code, assets and pubspec.yaml to the new project.

I was taking android studio template. flutter plugin. How to understand what exactly is the problem? Tried a solution from here https://github.com/flutter/flutter/issues/29608

build.grandle

   def localProperties = new Properties()
    def localPropertiesFile = rootProject.file('local.properties')
    if (localPropertiesFile.exists()) {
        localPropertiesFile.withReader('UTF-8') { reader ->
            localProperties.load(reader)
        }
    }

def flutterRoot = localProperties.getProperty('flutter.sdk')
if (flutterRoot == null) {
    throw new Exception("Flutter SDK not found. Define location with flutter.sdk in the local.properties file.")
}

def flutterVersionCode = localProperties.getProperty('flutter.versionCode')
if (flutterVersionCode == null) {
    flutterVersionCode = '1'
}

def flutterVersionName = localProperties.getProperty('flutter.versionName')
if (flutterVersionName == null) {
    flutterVersionName = '1.0'
}

apply plugin: 'com.android.application'
apply plugin: 'kotlin-android'
apply from: "$flutterRoot/packages/flutter_tools/gradle/flutter.gradle"

android {
    compileSdkVersion 28

    sourceSets {
        main.java.srcDirs += 'src/main/kotlin'
    }

    lintOptions {
        disable 'InvalidPackage'
    }

    defaultConfig {
        // TODO: Specify your own unique Application ID (https://developer.android.com/studio/build/application-id.html).
        applicationId "fiftypony.flutter_plugin_example"
        minSdkVersion 20
        targetSdkVersion 28
        versionCode flutterVersionCode.toInteger()
        versionName flutterVersionName
    }

    buildTypes {
        release {
            // TODO: Add your own signing config for the release build.
            // Signing with the debug keys for now, so `flutter run --release` works.
            signingConfig signingConfigs.debug
        }
    }
}

flutter {
    source '../..'
}

dependencies {
    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
    classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
    classpath 'com.android.tools.build:gradle:4.0.2'
    classpath 'com.google.gms:google-services:4.3.4'
}

flutter doctor

[√] Flutter (Channel stable, 1.20.4, on Microsoft Windows [Version 10.0.18363.1256], locale ru-RU)
    • Flutter version 1.20.4 at I:\flutter
    • Framework revision fba99f6cf9 (4 months ago), 2020-09-14 15:32:52 -0700
    • Engine revision d1bc06f032
    • Dart version 2.9.2


[√] Android toolchain - develop for Android devices (Android SDK version 30.0.3)
    • Android SDK at I:\game\rpg\ext3
    • Platform android-30, build-tools 30.0.3
    • Java binary at: C:\Program Files\Android\Android Studio\jre\bin\java
    • Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b01)
    • All Android licenses accepted.

[√] Android Studio (version 4.0)
    • Android Studio at C:\Program Files\Android\Android Studio
    • Flutter plugin version 49.0.2
    • Dart plugin version 193.7547
    • Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b01)