how to generate .aab from project flutter

i want to generate an .aab file from flutter project using this command

flutter build appbundle

 Building with sound null safety 

Removed unused resources: Binary resource data reduced from 1224KB to 1154KB: Removed 5%
Running Gradle task 'bundleRelease'...                             47.8s
✓ Built build/app/outputs/bundle/release/app-release.aab (21.7MB).

to put it in play.google.com but i get this problem

You uploaded an APK or Android App Bundle that was signed in debug mode. You need to sign your APK or Android App Bundle in release mode

so in the file android/app/build.gradle i did some modification

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.release
    }
}

but i get the same problem the .aab is in debug mode and not in release mode how to do it to upload the application in play store ?