Firebase AppCheck Error 403: App attestation failed

please 1st verify that you have init the app check correctely.

Initialize App Check

Add the following initialization code to your app so that it runs
before you use any Firebase services such as Storage, but after
calling Firebase.initializeApp();

import 'package:flutter/material.dart';
import 'package:firebase_core/firebase_core.dart';

// Import the firebase_app_check plugin
import 'package:firebase_app_check/firebase_app_check.dart';

Future<void> main() async {
  WidgetsFlutterBinding.ensureInitialized();
  await Firebase.initializeApp();
  await FirebaseAppCheck.instance.activate(
    // You can also use a `ReCaptchaEnterpriseProvider` provider instance as an
    // argument for `webProvider`
    webProvider: ReCaptchaV3Provider('recaptcha-v3-site-key'),
    // Default provider for Android is the Play Integrity provider. You can use the "AndroidProvider" enum to choose
    // your preferred provider. Choose from:
    // 1. Debug provider
    // 2. Safety Net provider
    // 3. Play Integrity provider
    androidProvider: AndroidProvider.debug,
    // Default provider for iOS/macOS is the Device Check provider. You can use the "AppleProvider" enum to choose
        // your preferred provider. Choose from:
        // 1. Debug provider
        // 2. Device Check provider
        // 3. App Attest provider
        // 4. App Attest provider with fallback to Device Check provider (App Attest provider is only available on iOS 14.0+, macOS 14.0+)
    appleProvider: AppleProvider.appAttest,
  );
  runApp(App());
}

if you believe that you have init the app check correctely thel try the following.

1. verify that google-services.json correctly placed in the project.

2. try to add SHA-256 key hashes in the firebase console also try by removing and adding it again.

also it may be helpful to go through following

Use App Check with the debug provider with Flutter

After you have registered your app for App Check, your app normally
won’t run in an emulator or from a continuous integration (CI)
environment, since those environments don’t qualify as valid devices.
If you want to run your app in such an environment during development
and testing, you can create a debug build of your app that uses the
App Check debug provider instead of a real attestation provider.